MgF2 system fluoride sintered body for radiation moderator and method for producing the same

ABSTRACT

A MgF 2  system fluoride sintered body for a radiation moderator having a compact polycrystalline structure excellent in radiation moderation performance, especially neutron moderation performance, contains CaF 2  of 90% by weight at the maximum and has a relative density of 95.2% or more.

TECHNICAL FIELD

The present invention relates to a MgF₂ system fluoride sintered bodyfor a radiation moderator and a method for producing the same, and moreparticularly, to a MgF₂ system fluoride sintered body for a radiationmoderator having a compact structure suitable for a moderator torestrict the radiation velocity and energy of radioactive rays of everykind such as neutrons and a method for producing the same.

BACKGROUND ART

Among fluorides, a calcium fluoride (CaF₂) single crystal body, amagnesium fluoride (MgF₂) single crystal body and the like have beenused in the optical field limited to, for example, the vacuumultraviolet region of wavelengths of 160 nm or less, or the extremeinfrared region of wavelengths of 3 μm or more. Fluorides have been usedas optical materials such as a window material, a lens and a prism forsuch specific wavelength regions wherein light does not pass throughhigh-purity quart glasses and optical glasses widely used in the market.Therefore, the amount of demand thereof is naturally small.

Among them, a magnesium fluoride (MgF₂) noticeably shows a birefringencephenomenon, and therefore, it is not suitable for a lens or a prism. Itis rarely used as a window material for transmitted light, and thedemand thereof is very small.

In order to manufacture a MgF₂ single crystal body for optical use whichis substantially only one usage thereof, the Bridgman-Stockbarger method(crucible descending method) being a kind of so-called zone melting, orthe Czochralski method (single crystal pulling method) is adopted.

In the Bridgman-Stockbarger method, wherein a vertical heating furnaceis used, a crucible is charged with a raw material powder. The crucibleis allowed to slowly descend from the upper side in the heating furnaceand pass through a soaking zone, by which the state of the raw materialin the crucible is shifted from ‘heating’ to ‘melting’, and then to‘solidification (crystallization)’, and then to ‘crystal growth’,gradually from the lower portion thereof toward the upper portionthereof. Meanwhile, vaporized gases generated through sublimation of theraw material or gases existing in voids among raw material particles arereleased from the top side of the crucible into the furnace so as toreduce residual bubbles in the crystal generated in solidification.

In the Czochralski method, wherein a heating furnace and a crucible areused, a raw material in the crucible is melted. A seed crystal is dippedfrom the upper side of the crucible in the upper surface portion of themelt, and the melt of the dipped portion is slowly pulled up withrotation and solidified (crystallized) so as to pull a single crystalupward.

Both of these methods are appropriate to reduce residual bubbles in thecase of a raw material such as a fluoride which easily sublimate in theheating process. However, the above-described crystal growth takes along time, for example, it takes several months to produce a singlecrystal of about 10 kg. Thus, the productivity thereof is extremely low,resulting in very high producing cost.

Generally speaking, there are very few cases wherein a fluoride is usedfor other than such optical uses. The CaF₂ single crystal body, or asingle crystal body of lithium fluoride (LiF) and aluminum fluoride(AlF₃) has been rarely used as a shield to neutrons, one of radioactiverays. However, such single crystal bodies have plane orientationdependency of moderation performance originated in crystal orientationand ununiformity due to structural defects such as a subgrain or asubboundary. Moreover, in the case of MgF₂, it has a largebirefringence. Therefore, it is necessary to distinguish eachcharacteristic of these materials for proper use, resulting indifficulties in use.

Since the above fluoride single crystal bodies include a defect portionsuch as a subgrain or a subboundary having a loose crystal structure anda slightly low density, in many cases, the density thereof is slightlylower than the theoretical density (i.e. true density). For example, inthe case of MgF₂, the true density thereof is 3.15 g/cm³, while thedensities of actual single crystals are 3.130-3.145 g/cm³. Many of themhave a relative density of 99.4%-99.8%.

Hereinafter, radioactive rays in relation to the usage of the presentinvention are briefly described. The radioactive rays are roughlyclassified into alpha (α)-rays, beta (β)-rays, gamma (γ)-rays, X-rays,and neutrons. The power passing through a substance (penetrability)gradually increases in this order.

The neutrons which have the highest penetrability among them are furtherclassified into the below-described groups, for example, according tothe energy level which they have. The energy level each type of neutronshas is shown in parentheses, and the larger the value is, the higher thepenetrability is. In the order of the lowest penetrability, they areclassified into cold neutrons (up to 0.002 eV), thermal neutrons (up to0.025 eV), epithermal neutrons (up to 1 eV), slow neutrons (0.03-100eV), intermediate neutrons (0.1-500 keV) and fast neutrons (500 keV ormore).

Here, there are various views concerning the classification of neutrons,and the energy values in the parentheses are not precise. For example,there is a view that mentions 40 KeV or less, which is within the aboveenergy region of intermediate neutrons, as the energy level ofepithermal neutrons.

The typical effective utilization of neutrons is an application to themedical care field. In particular, the radiation therapy in which tumorcells such as malignant cancers are irradiated with neutrons so as to bebroken has been coming into general use in recent years. However, inorder to obtain medical effects in the present radiation therapy,neutrons of a certain high energy level must be used, so that in theirradiation of neutrons, the influence on a healthy part other than anaffected part of a patient cannot be avoided, leading to side effects insome cases. Therefore, in the present situation, the application of theradiation therapy is limited to severe patients.

When a normal cell is exposed to high-energy-level neutrons, its DNA isdamaged, leading to side effects such as dermatitis, anemia due toradiation and leukopenia. Furthermore, in some cases, a late injury maybe caused some time after treatment, and a tumor may be formed and bleedin the rectum or the urinary bladder.

In recent years, in order not to cause such side effects and lateinjuries, methods of pinpoint irradiation on a tumor have been studied.Examples thereof are: “Intensity Modulated Radiation Therapy (IMRT)” inwhich a tumor portion is three-dimensionally irradiated accurately witha high radiation dose; “Motion Tracking Radiation Therapy” in whichradiation is emitted to motions in the body of a patient such asbreathing or heartbeat; and “Particle Beam Radiation Therapy” in which abaryon beam or a proton beam each having a high remedial value isintensively emitted.

As a characteristic of a neutron, the half-life thereof is short, about15 min. The neutron decays in a short period of time, releases electronsand neutrinos, and turns into protons. And the neutron has no charge,and therefore, it is easily absorbed when it collides with a nucleus.The absorption of neutrons in such a manner is called neutron capture,and one example of an application of neutrons to the medical care fieldby use of this feature is the below-described “Boron Neutron CaptureTherapy (hereinafter, referred to as BNCT)”, a new cancer therapy whichis recently gaining attention.

In this BNCT, by causing tumor cells such as malignant cancers to reactwith a boron drug which is injected into the body by an injection, areaction product of a boron compound is formed in the tumor portion. Thereaction product is then irradiated with neutrons of an energy levelwhich has less influences on a healthy part of the body (desirablycomprising mainly epithermal neutrons, and low-energy-level neutronsbeing lower than epithermal neutrons). And a nuclear reaction with theboron compound is caused only within a very small range, resulting inmaking only the tumor cells extinct. Originally, cancer cells easilytake boron into them in the process of vigorously increasing, and in theBNCT, by use of this feature, only the tumor portion is effectivelybroken.

This method has been attracting attention as an excellent radiationtherapy since quite long before because of small influences on a healthypart of a patient, and has been researched and developed in variedcountries. However, there are wide-ranging important problems on thedevelopment such as development of a neutron generator and a device fora selection of the types of neutrons to be remedially effective, andavoidance of influences on a healthy part other than an affected part ofa patient (that is, formation of a boron compound only in a tumorportion). Therefore, the method has not come into wide use as a generaltherapy.

Significant factors in terms of apparatus why it has not come into wideuse are necessity to develop a neutron generating device for exclusiveuse, insufficient downsizing of the whole apparatus including thegenerating device and insufficient enhancement of its performance.

For example, there is a latest system of the BNCT, which a group withKyoto University as the central figure has been promoting (Non-PatentDocument 1 and Non-Patent Document 2). This system comprises anapparatus for medical use only, having a cyclotron accelerator as aneutron generator which is exclusively installed without being attachedto an existing nuclear reactor. One report says that the acceleratoralone weighs about 60 tons, and its size is large. In the cyclotronsystem, protons are accelerated by use of a centrifugal force in acircular portion of the cyclotron. Accordingly, in order to efficientlygenerate neutrons, it is required to make the diameter of the circularportion large so as to obtain a large centrifugal force. That is one ofthe reasons why the apparatus is large.

Furthermore, in order to safely and effectively utilize the generatedradiation (mainly neutrons), a radiation shield such as a shieldingplate (hereinafter, referred to as a moderator) is required. Asmoderators, CaF₂ and polyethylene containing LiF, as well as Pb, Fe, Aland polyethylene, are selected. It cannot be said that the moderationperformance of these moderators is sufficient, and in order to realizerequired moderation, the moderator becomes quite thick. Therefore, themoderation system device portion including the moderator is also one ofthe reasons why the apparatus is large.

In order to allow this BNCT to come into wide use in general hospitals,downsizing of the apparatus is necessary. In addition to furtherdownsizing of the accelerator, to improve the remedial values bydeveloping a moderator having high moderation performance and achievedownsizing of the moderation system device by the improvement ofmoderation performance is an urgent necessity.

The moderator which is important for downsizing a BNCT apparatus andimproving remedial values is briefly described below.

As described above, in order to safely and effectively utilizeradiation, it is necessary to arrange a moderator having the rightperformance in the right place. In order to effectively utilize neutronshaving the highest penetrability among radioactive rays, it is necessaryto accurately know the moderation performance of every kind ofsubstances to neutrons so as to conduct effective moderation.

One example of the selection of particle beam types in order toeffectively utilize neutrons for medical care is shown below. Byremoving high-energy neutrons which adversely influence the body (suchas fast neutrons and a high-energy part of intermediate neutrons) asmuch as possible, and by further reducing extremely-low-energy neutronshaving little medical effect (such as thermal neutrons and coldneutrons), the ratio of neutrons having high medical effects (such as alow-energy part of intermediate neutrons and epithermal neutrons) isincreased. As a result, a particle beam effectively utilized for medicaltreatment can be obtained.

The low-energy part of intermediate neutrons and epithermal neutronshave a relatively high invasive depth to the internal tissues of apatient. Therefore, for example, in the case of irradiating the headwith the low-energy part of intermediate neutrons and epithermalneutrons, as far as the tumor is not present in a considerably deeppart, without craniotomy required, it is possible to carry out effectiveirradiation to an affected part in an unopened state of the head.

On the other hand, when the extremely-low-energy neutrons such asthermal neutrons are used in an operation, because of their low invasivedepth, craniotomy is required, resulting in a significant burden on thepatient.

In order to improve remedial values in the BNCT, it is required toirradiate an affected part with a large quantity of neutrons comprisingmainly epithermal neutrons and some thermal neutrons. Specifically, anestimated dose of epithermal neutrons and thermal neutrons required incases where the irradiation time is in the order of one hour, is about1×10⁹ [n/cm/sec]. In order to secure the dose, it is said that as theenergy of an outgoing beam from an accelerator being a source ofneutrons, about 5 MeV-10 MeV is required when beryllium (Be) is used asa target for the formation of neutrons.

The selection of particle beam types through moderators of every kind ina neutron radiation field for BNCT using an accelerator is describedbelow.

A beam emitted from the accelerator collides with a target (Be, in thiscase), and by nuclear reaction, high-energy neutrons (fast neutrons) aremainly generated. As a method for moderating the fast neutrons, usinglead (Pb) and iron (Fe) each having a large inelastic scattering crosssection, the neutrons are moderated to some extent. In order to furthermoderate the neutrons moderated to some extent (approximately, up to 1MeV), optimization of the moderator according to the neutron energyrequired in the radiation field is conducted.

As a moderator, aluminum oxide (Al₂O₃), aluminum fluoride (AlF₃),calcium fluoride (CaF₂), graphite, heavy water (D₂O) or the like isgenerally used. By injecting the neutrons moderated nearly to 1 MeV intothese moderators, they are moderated to the epithermal neutron region ofthe energy suitable for BNCT (4 keV-40 keV).

In the case of the above Non-Patent Document 1 and Non-Patent Document2, as moderators, Pb, Fe, polyethylene, Al, CaF₂ and polyethylenecontaining LiF are used. The polyethylene and polyethylene containingLiF among them are used as moderators for safety (mainly for shielding)which cover the outside portion of the apparatus in order to preventleakage of high-energy neutrons out of the radiation field.

It can be said that it is appropriate to moderate the high-energy partof neutrons to some extent using Pb and Fe among these moderators (thefirst half of the stage of moderation), but it cannot be said that thesecond half of the stage of moderation using Al and CaF₂ after themoderation to some extent is appropriate. That is because the moderatorsused in the second half of the stage thereof have insufficient shieldingperformance to fast neutrons, and a high ratio of fast neutrons having apossibility of bad influences on healthy tissues of a patient is left inthe moderated neutron type.

By reason of CaF₂ having insufficient shielding performance to thehigh-energy part of neutrons as a moderator used in the second half ofthe stage thereof, part of them passes without being shielded. Thepolyethylene containing LiF used with CaF₂ in the second half of thestage thereof covers over the entire surface except an outlet ofneutrons on the treatment room side. It is arranged so as to preventwhole-body exposure of a patient to the fast neutrons, without having afunction as a moderator on the outlet of neutrons. For information, thepolyethylene among the moderators in the first half of the stage thereofcovers over the entire surface of the periphery of the apparatus exceptthe treatment room side, like the polyethylene containing LiF in thesecond half of the stage thereof, and it is arranged so as to preventthe fast neutrons from leaking to the surroundings of the apparatus.

Therefore, instead of CaF₂ as a shielding member to fast neutrons in thesecond half of the stage thereof, the development of a moderator whichcan shield and moderate high-energy-level neutrons while suppressing theattenuation of intermediate-energy-level neutrons required for treatmenthas been desired.

From various kinds of researches/studies, the present inventors found aMgF₂ sintered body or MgF₂ system substances, for example, a MgF₂—CaF₂binary system sintered body as a moderator which makes it possible toobtain neutrons (neutrons of the energy of 4 keV-40 keV) mainlycomprising epithermal neutrons in anticipation of the highest remedialvalue, from the above neutrons moderated to some extent (the energythereof is approximately up to 1 MeV). As the MgF₂ system substances, aMgF₂—LiF binary system sintered body, a MgF₂—CaF₂—LiF ternary systemsintered body other than the MgF₂—CaF₂ binary system sintered body canbe exemplified.

For information, as of now, there has been no report that magnesiumfluoride (MgF₂) was used as a moderator to neutrons, not to mention thatthere has been no report that a MgF₂ sintered body or a MgF₂—CaF₂ binarysystem sintered body was used as such neutron moderator.

The present inventors have filed an application of an invention relatingto a sintered body of MgF₂ simple (a technical term related to rawmaterial technology, a synonym for “single”) prior to this invention(Japanese Patent Application No. 2013-142704, hereinafter, referred toas Prior Application I). Furthermore, the present inventors have filedan application of an invention relating to a MgF₂—CaF₂ binary systemsintered body as a neutron moderator (Japanese Patent Application No.2014-193899, hereinafter, referred to as Prior Application II). ThePrior Application I and Prior Application II are described later indetail.

The present invention was achieved in order to further improve thecharacteristics of the MgF₂ sintered body and the MgF₂—CaF₂ binarysystem sintered body according to the two prior applications.

A single crystal body of MgF₂ has high transparency, and high lighttransmittance within a wide range of wavelengths of 0.2 μm-7 μm, and ithas a wide band gap and high laser resistance. Therefore, it has beenmainly used as a window material for eximer laser. Or when a MgF₂ singlecrystal body is deposited on the surface of a lens, it shows effects ofprotection of the inner parts thereof or prevention of irregularreflection. In either case, it is used for optical use.

On the other hand, since the MgF₂ sintered body has low transparencybecause of its polycrystalline structure, it is never used for opticaluse. Since the MgF₂ sintered body has high resistance to fluorine gasand inert gas plasma, a few applications concerning an applicationthereof to a plasma-resistant member in the semiconductor producingprocess have been filed. However, there is no publication or report thatit was actually used in the semiconductor producing process.

That is because the Bridgman-Stockbarger method (crucible descendingmethod) or the Czochralski method (single crystal pulling method) isadopted in order to manufacture a MgF₂ single crystal body, as describedabove, resulting in the MgF₂ single crystal body's strong image ofextremely high price, and because a MgF₂ sintered body produced by ageneral sintering method has a low density due to the foaming propertyof the MgF₂ raw material, leading to a tendency to have low mechanicalstrength, as described in the below-mentioned Patent Document 1.

As for a MgF₂ sintered body, according to the Japanese PatentApplication Laid-Open Publication No. 2000-302553 (the below-mentionedPatent Document 1), the greatest defect of ceramic sintered bodies offluoride such as MgF₂, CaF₂, YF₃ and LiF is low mechanical strength. Andin order to solve this problem, the invention was achieved, whereinsintered bodies compounded by mixing these fluorides with alumina(Al₂O₃) at a predetermined ratio can keep excellent corrosion resistanceof the fluorides as well as obtain high mechanical strength.

However, as for the corrosion resistance and mechanical strength of thesintered bodies produced by this method, in any combination, thesintered bodies are simply allowed to have just an intermediatecharacteristic between the characteristic of any of the fluorides andthat of alumina. No sintered body having a characteristic exceedingone's characteristic superior to the other's has been obtained bycompounding. In addition, their use is limited to high corrosionresistance uses, different from the uses of the present invention.

Another sintered body mainly comprising MgF₂ is described in JapanesePatent Application Laid-Open Publication No. 2000-86344 (thebelow-mentioned Patent Document 2), but its use is also limited to aplasma-resistant member. In the Patent Document 2, a sintered bodycomprises a fluoride of at least one kind of alkaline earth metalsselected from the group of Mg, Ca, Sr and Ba, in which the total amountof metallic elements other than the alkaline earth metals is 100 ppm orless on a metal basis, the mean particle diameter of crystal grains ofthe fluoride is 30 μm or less, and the relative density is 95% or more.

However, the materials in the list (Table 1) of Examples of the PatentDocument 2 were obtained by firing a fluoride of each single kind of theabove four alkaline earth metals (i.e. MgF₂, CaF₂, SrF₂ and BaF₂), andno fired mixture of those fluorides is described.

Still another example of an application of a sintered body mainlycomprising MgF₂ to a plasma-resistant member is the Japanese PatentApplication Laid-Open Publication No. 2012-206913 (the below-mentionedPatent Document 3). The Patent Document 3 discloses an inventionwherein, since a sintered body of MgF₂ simple has a defect of lowmechanical strength, by mixing at least one kind of non-alkalinemetallic dispersed particles having a lower mean linear thermalexpansion coefficient than MgF₂ such as Al₂O₃, AlN, SiC or MgO, thedefect of low mechanical strength thereof can be compensated for. When asintered body of such mixture is used as the above moderator toneutrons, the moderation performance thereof is greatly different fromthat of MgF₂ simple because of the influence of the non-alkaline metalmixed into MgF₂. Therefore, it is predicted that it is difficult toapply a sintered body of this kind of mixture to a use as a moderator.

In addition, an example of an application of a sintered body mainlycomprising CaF₂ to a plasma-resistant member is the Japanese PatentApplication Laid-Open Publication No. 2004-83362 (the below-mentionedPatent Document 4). The Patent Document 4 describes a method whereinusing hydrofluoric acid, impurities other than Mg are removed from alow-purity raw material containing Mg, so as to precipitate high-purityCaF₂, and a fluoride sintered body whose starting raw material is thehigh-purity CaF₂ containing Mg of 50 ppm or more and 5% by weight orless is produced. A problem here is a state of Mg contained in thestarting raw material, though the state is not described at all. Andthere is no description concerning the technique by which the degree ofpurity of the low-purity raw material is raised using hydrofluoric acid.

Then, when presuming the process of raising the degree of purity of alow-purity raw material as a person skilled in the art, generallyspeaking, in the case of raising the degree of purity of a low-purityraw material using hydrofluoric acid, a method is often adopted, whereinimpurities in the raw material are first dissolved into a hydrofluoricacid solution as many as possible, and if a component (Ca, here) desiredto be a main raw material dissolved with impurities in this dissolutionprocess, the component is precipitated and separated by use of thedifference in solubility among the dissolved components. When furtherreviewing the invention, it is presumed that Mg exhibited differentdissolution behavior from other impurities. In the specification, it isreferred to as only “Mg”, and according to the descriptions of Examplesin Table 1, as for high concentrations of impurity components other thanMg (such as Fe, Al, Na and Y), all of their concentrations weredecreased by purity raising treatment, but only the concentration of Mgdid not change, being 2000 ppm before the treatment and being also 2000ppm after the treatment, leading to the above presumption. Hence, thereis a high possibility that Mg might be in a state which is hard todissolve in hydrofluoric acid, that is, a metal state. If CaF₂containing metal Mg is a starting raw material, the sintering processthereof is very different from the case like the present inventionwherein a mixture of CaF₂ and MgF₂ is a starting raw material, and thecharacteristics of the sintered bodies are also very different from eachother.

On the other hand, inventions relating to a neutron moderator weredisclosed lately. One of them is the Japanese Patent No. 5112105 (PatentDocument 5). The Patent Document 5 discloses ‘a moderator whichmoderates neutrons, comprising a first moderating layer obtained bymelting a raw material containing calcium fluoride (CaF₂), and a secondmoderating layer comprising metal aluminum (Al) or aluminum fluoride(AlF₃), the first moderating layer and the second moderating layer beingadjacent to each other’.

In the Patent Document 5, the first moderating layer obtained by meltingthe raw material containing CaF₂ is disclosed, but raw materialconditions such as the purity, components, particle size and treatmentmethod thereof, and melting conditions such as heating temperatures,holding times thereof and the type of heating furnace are not mentionedat all, very insincerely described as a patent specification. In thePatent Document 5, there is no description suggesting that somethingrelated to MgF₂ should be used as a neutron moderator.

Another invention is disclosed in the document by KONONOV, O. E. et al.(Non-Patent Document 4). This Non-Patent Document 4 says that MgF₂ hasgood moderation performance as a neutron moderator. However, there is nodescription about the MgF₂ used therein except for ‘its density of 3.14g/cm³’. Even how to produce it, conditions for producing it, and whetherthe mineral texture thereof is a single crystal or polycrystalline oramorphous (vitreous), or a mixture of them, are not mentioned at all.Furthermore, whether it was produced by the writers thereof or byoutsiders is not mentioned.

In cases where a polycrystalline article or an amorphous article whichis not on the market is used, generally the descriptions such as itsorigin and the characteristics/physical properties such as its qualityother than the density should be included. Therefore, in this case, itis presumed that a ‘single crystal’ article being on the market wasused.

In case where the writers made a polycrystalline article of MgF₂ on anexperimental basis by a sintering method, as described below, it shouldbe concluded that an article having an extremely high density (3.14g/cm³: relative density of 99.7%) could be made although MgF₂ has a highfoaming property, and that an article of extremely excellent qualitythat had never been seen in the past could be made. In such case,naturally the description of such excellent quality should be includedin the document. Hence, it can be concluded that as the MgF₂ article, a‘single crystal’ article available in the market or that one can producewas used.

In the preceding documents, as described above, there is no descriptionsuggesting the use of a sintered body of MgF₂ as a moderator toneutrons, one kind of radioactive rays. In such situation, the presentinventors found that it was possible to use a MgF₂ sintered body with amodification made thereto as a moderator to neutrons, one kind ofradioactive rays, and achieved the invention of the Prior Application I.

In the invention of the Prior Application I, a high-purity MgF₂ rawmaterial was pulverized and two-stage compressing and molding wasconducted thereon. That is, after molding by a uniaxial press moldingmethod, this press molded body was further molded by a cold isostaticpressing (CIP) method so as to form a CIP molded body. Then, by firingthe same in three steps with different heating conditions using anatmospheric furnace in which the atmosphere can be adjusted, a sinteredbody having a compact structure was produced while suppressing foamingof MgF₂ as much as possible. However, since MgF₂ very easily foams, itwas difficult to actually suppress foaming thereof. As a result, therange of the relative densities (i.e. 100×[bulk density of a sinteredbody]/[true density](%)) of the sintered bodies produced by this methodwas 92%-96%, and the mean value thereof was of the order of 94%-95%.

A characteristic desired for a sintered body for a neutron moderator is‘the mean value of relative densities of 95% or more at least, desirably96% or more should be stably secured’.

As described above, the fundamental performance required for a moderatorused in the BNCT method is ‘to prevent high-energy neutrons such as fastneutrons from leaking, and to sufficiently secure epithermal neutronsnecessary for therapy’, and a sintered body satisfying the abovecharacteristic may be a sintered body having the above fundamentalperformance.

It could not be said that the relative density of the sintered bodyaccording to the Prior Application I should lead to the performancethereof sufficient as a moderator used in the BNCT method. Therefore,the present inventors worked toward further development and found outthat when using a MgF₂—CaF₂ binary system sintered body for radiationuse, the relative density thereof could be easily improved, comparedwith a sintered body of MgF₂ simple, resulting in an anticipation toimprove the performance as a moderator, which led to the invention ofthe Prior Application II.

In the Prior Application II, after mixing/pulverizing a high-purity MgF₂raw material and a CaF₂ raw material, two-stage compressing and moldingwas conducted thereon. That is, after molding the same by a uniaxialpress molding method, this press molded body was further molded by acold isostatic pressing (CIP) method so as to form a CIP molded body.Then, it was fired in three steps with different heating conditionsusing an atmospheric furnace in which the atmosphere can be adjusted soas to produce a sintered body having a compact structure whilesuppressing foaming of MgF₂ as much as possible.

However, since a MgF₂—CaF₂ binary system molded body also easily foams,it was difficult to actually suppress foaming thereof. Consequently, therange of the relative densities of the sintered bodies produced by thismethod was 94%-97%, and the mean value thereof was of the order of95%-96%.

It was a level in which the request of ‘the mean value of relativedensities of 95% or more at least, as a moderator’ was just cleared, andit was difficult to satisfy the request ‘desirably the mean value ofrelative densities of 96% or more should be stably secured’.

This MgF₂—CaF₂ binary system sintered body had a relative density about1% higher than the MgF₂ sintered body in the Prior Application I.However, it was admitted that open pores remained in the peripheryportion of the sintered body, judging from a phenomenon in which purewater penetrated the sintered body when it was soaked in pure water formeasuring the bulk density thereof. Thus, even in the invention of thePrior Application II achieved in order to improve the invention of thePrior Application I, it was predicted that open pores still remained inthe sintered body, and it was proved that there should be yet room forimprovement.

As described above, since MgF₂ very easily foam, it is not easy toactually suppress foaming thereof. Consequently, it is desired that eventhe sintered bodies produced by the methods according to the PriorApplication I and Prior Application II should have further improvedmoderation performance to radiation, especially to neutrons, and ahigher relative density (i.e. 100×[bulk density of the sinteredbody]/[true density](%)).

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Patent Application Laid-Open Publication    No. 2000-302553-   Patent Document 2: Japanese Patent Application Laid-Open Publication    No. 2000-86344-   Patent Document 3: Japanese Patent Application Laid-Open Publication    No. 2012-206913-   Patent Document 4: Japanese Patent Application Laid-Open Publication    No. 2004-83362-   Patent Document 5: Japanese Patent No. 5112105

Non-Patent Document

-   Non-Patent Document 1: H. Tanaka et al., Applied Radiation and    Isotopes 69 (2011) 1642-1645-   Non-Patent Document 2: H. Tanaka et al., Applied Radiation and    Isotopes 69 (2011) 1646-1648-   Non-Patent Document 3: Hiroaki Kumada, Tetsuya Yamamoto, Dose    Evaluation of Neutron Capture Therapy in JRR-4, Health Physics,    42(1), (2007) 23-37-   Non-Patent Document 4: KONONOV, O. E. et al., “ACCELERATOR-BASED    SOURCE OF EPITHERMAL NEUTRONS FOR NEUTRON CAPTURE THERAPY”, Atomic    Energy, November, 2004, Vol. 97, No. 3, pp. 626-631

SUMMARY OF THE INVENTION Solution to Problem and Advantageous Effect ofInvention

The present invention was developed in order to solve the aboveproblems, and it is an object of the present invention to provide a MgF₂system fluoride sintered body for a radiation moderator, havingexcellent characteristics as a moderator used for moderating the energyof neutrons, a kind of radiation, in the good use of the neutrons fortherapy, which makes it possible to enhance remedial values and downsizean apparatus for therapy, and which is inexpensive unlike a singlecrystal body, and a method for producing the same.

In other words, it is an object of the present invention to provide aMgF₂ system fluoride sintered body for a radiation moderator having avery compact structure, without plane orientation dependency of themoderation performance originated in crystal orientation which a singlecrystal body has, and without ununiformity based on the structuraldefects such as subgrains, and a producing method by which such sinteredbodies can be stably produced.

The MgF₂ system fluoride sintered body according to the presentinvention is mentioned mainly as a moderator to neutrons, but thesintered body has excellent performance as a member for shielding to notonly neutrons but also other radioactive rays such as X-rays orgamma-rays.

In the Prior Application I and Prior Application II, the presentinventors gave basic consideration to the selection of moderators(material substances) suitable for shielding (i.e. moderation) toneutrons, and found a MgF₂ system fluoride sintered body, specifically aMgF₂ sintered body and a MgF₂—CaF₂ binary system sintered body.

As the basic characteristics required for a moderator other thanmoderation performance, a characteristic of keeping the shape of theproduct is exemplified. It is important to be excellent in mechanicalstrength with which damage in mechanical processing and in handlingduring manufacturing the product can be prevented.

The mechanical strength of a sintered body is determined by microstrength of bonding parts between particles, the compactness of thesintered body, and moreover, the brittleness originated from a crystalstructure (such as polycrystal or single crystal or amorphous) of theparent thereof.

The compactness of the sintered body is determined by the defoamingstate such as the sizes, shapes, distribution and number of bubbles, inother words, the shape such as the width and length of the bonding partsand a bound body (parent) of ex-particles.

Basic technical ideas of the present invention are:

(1) relaxing the sintering conditions by preparing a starting rawmaterial to satisfy a proper particle size condition so as to make itpossible to conduct low-temperature sintering, that is, increasing thepacking density by making the particle sizes of the starting rawmaterial smaller than those in the Prior Application I and PriorApplication II and setting a smaller range of particle sizes, leading topromoting the sintering reaction, particularly flocculation of particlesthrough solid phase reaction and bonding thereof (low-temperaturesintering) (mainly a phenomenon in the preliminary sintering step andprimary sintering step);

(2) allowing the sintered body after the completion of this primarysintering to be a high-density sintered body through the formation of asolid solution, having strong cohesion between particles;

(3) when a fluoride system raw material is heated at a high temperature,part of the raw material vaporizes (mainly part of the fluoride isthermally decomposed (sublimates) and generates fluorine gas), leadingto the formation of bubbles (foaming) By sintering at a temperature lowenough to avoid this foaming and making the heating process (sinteringheat pattern) proper, a compact sintered body is obtained; and

(4) although an atmospheric sintering method is adopted both in theprimary and secondary sintering steps in the Prior Application I andPrior Application II, a pressure sintering method such as a uniaxial hotpress method or a hot isostatic pressing (HIP) method is adopted both inthe primary and secondary sintering steps as needed in the presentinvention. Furthermore, another option of conducting the tertiarysintering step is added thereto, wherein a pressure sintering methodsuch as the uniaxial hot press method or the hot isostatic pressing(HIP) method is adopted so as to further reduce bubbles, voids and thelike in the sintered body, resulting in a more compact sintered body.

By combining the above technical ideas (1)-(4), the present inventionaimed to stably produce MgF₂ system fluoride sintered bodies for aradiation moderator excellent in moderation performance required as amember for a moderator to radiation, especially to neutrons andmechanical strength (shape keeping) characteristic important as afundamental characteristic other than the moderation performance.

Concerning the technical idea (1), by making the particle size conditionof the starting raw material proper, low-temperature sintering waspromoted.

Using a high-purity MgF₂ raw material powder and a CaF₂ raw materialpowder, the mean diameter of which is about 140 μm in median diameter(hereinafter, simply referred to as median diameter), the MgF₂ rawmaterial powder was pulverized by the below-mentioned pulverizationmethod in the case of MgF₂ simple. On the other hand, in the case ofMgF₂—CaF₂ binary system, a prescribed quantity of MgF₂ raw materialpowder and that of CaF₂ raw material powder each were scaled, mixedusing a V mixer and pulverized by the below-described pulverizationmethod. Thereafter, the same was provided to the subsequent processingstep, and the characteristic evaluation of the completed sintered bodywas conducted so as to find proper particle size conditions of thestarting raw material.

As a result,

(i) the particle size distribution range should be small, specifically,the maximum particle diameter should be 50 μm or less, desirably 30 μmor less;

(ii) as a preferable state of the particle size distribution, the shapeof the particle size distribution curve drawn with particle diameter(μm) as the abscissa and particle diameter ratio (ratio of everyparticle diameter: wt. %) as the ordinate, should be not ‘2-peak type’or ‘3-peak type’, but ‘sub-1-peak type’ or ‘1-peak type’, preferably1-peak type which closely resembles that of normal distribution;

(iii) the median diameter should be 6 μm or less, desirably 3 μm orless; and

by simultaneously satisfying these conditions of the items (i), (ii) and(iii), the sintered body could be allowed to have a high density,leading to a noticeable improvement of moderation performance as aneutron moderator as shown below.

The foaming phenomenon in the above technical idea (3) is describedbelow a little in detail. Using a differential thermal analyzer,alterations in weight and in endothermic and exothermic amount of thesample of the starting raw material were examined while heating. As aresult, a minute quantity of weight decrease was found at approximately800° C.-850° C., though there were slight differences depending on themix proportion of the starting raw material. It appears that fluorineattached to a parent of a preliminary sintered body or fluorineresolving in the parent, for example, with a weak bonding property,dissociated and decomposed first of all. After further heating, a pointof inflection of the weight decrease curve appeared at approximately850° C.-900° C., and the weight decrease became noticeable.

The results of this differential thermal analysis, and the examinationresults of the sintering conditions and the structure of the sinteredbody in the below-mentioned preliminary sintering test, specifically,the examination results such as:

1. the situation of bubble generation in the sintered body;

2. the situation of organizational structure of the sintered portion;and

3. the bulk density of the sintered body,

were comprehensively considered. It was anticipated that when heated ata temperature of the point of inflection of the weight decrease curve orhigher, part of bonded fluorine element in MgF₂ or CaF₂ would start todecompose, and cause the generation of fluorine gas, leading to theformation of fine bubbles.

Then, the temperature of the point of inflection of the weight decreasecurve, 850° C.-900° C. is referred to as the starting temperature offoaming (Tn). The temperature at which vaporization started was slightlydifferent depending on the composition. In the case of a compositionmainly comprising MgF₂ (MgF₂ of 70-99.8% by weight, and CaF₂ of therest), vaporization started at about 800° C. and it became quite briskat about 850° C. (the point of inflection, that is, the startingtemperature of foaming Tn was decided to be 850° C.). In the case of acomposition mainly comprising CaF₂ (MgF₂ of 10-40% by weight, and CaF₂of the rest), vaporization started at about 850° C. and it became quitebrisk at about 900° C. (similarly, Tn was decided to be 900° C.). In thecase of MgF₂ of 40-70% by weight and CaF₂ of the rest, vaporizationstarted at around the intermediate temperature between the above twocases, that is, in the temperature limits of about 825° C. or more, andit became quite brisk at about 875° C. (similarly, Tn was decided to be875° C.).

That is, in the case of the composition mainly comprising MgF₂ (MgF₂ of70-99.8% by weight, and CaF₂ of the rest), sublimation starts at about800° C., and it becomes brisk and foaming starts at about 850° C. In thecase of the composition mainly comprising CaF₂ (MgF₂ of 10-40% byweight, and CaF₂ of the rest), sublimation starts at about 850° C. andit becomes brisk and foaming starts at about 900° C. In the case of MgF₂of 40-70% by weight, and CaF₂ of the rest, the mix proportionintermediate therebetween, sublimation starts at about 825° C., and itbecomes brisk and foaming starts at about 875° C.

Thus, when a fluoride sublimates (a phenomenon in which a solid phasechanges into a gas phase without passing through a liquid phase. In thiscase, a synonym for “vaporize”), fluorine gas is generated, resulting ingeneration of fine bubbles in the sintered body.

When observing the broken-cross section of the sintered body with anelectron microscope (SEM), the sizes of the bubbles range from smallones of several μm to large ones of 20 μm-40 μm in diameter seen on thebroken-cross section.

The shapes of the small ones of several μm are approximately circles andthe shapes of the large ones are rarely circles. Most of them areirregular such as long and narrow, or angular.

Judging from these shapes, it is considered that the small ones arebubbles which have just been generated through sublimation of the abovefluoride, and that the large ones are aggregates of some of thegenerated bubbles or residuals originated from voids among particles orthe like which could not defoam in the sintering process.

The reason why the value of relative density is shown by rangecorresponding to one value of bulk density is because in the case of abinary system sintered body of MgF₂ and CaF₂, the true densities of theboth are different (that of MgF₂ is 3.15 g/cm³, while that of CaF₂ is3.18 g/cm³), and therefore, depending on the mix proportion thereof, thetrue density of the mixture varies slightly. Here, the value of truedensity of the mixture is decided as shown below, so as to calculate therelative density thereof.

It is decided that:

(a) the true density is 3.15 g/cm³, in the case of a composition mainlycomprising MgF₂, that is, MgF₂ of 70% by weight or more and 99.8% byweight or less (referred to as 70-99.8% by weight in the presentapplication) and CaF₂ of the rest;

(b) the true density is 3.16 g/cm³, in the case of MgF₂ of 40% by weightor more and less than 70% by weight (referred to as 40-70% by weight)and CaF₂ of the rest; and

(c) the true density is 3.17 g/cm³, in the case of MgF₂ of 10% by weightor more and less than 40% by weight (referred to as 10-40% by weight)and CaF₂ of the rest.

Concerning the technical idea (4), in the primary and secondarysintering steps, an atmospheric firing method using an atmosphericfiring furnace in which the atmosphere can be adjusted was adopted as abasic technique, similarly to the Prior Application I and PriorApplication II.

In the primary and secondary sintering steps, in place of thisatmospheric firing method, a hot press method using a uniaxial pressinghot press furnace and a hot isostatic pressing (HIP) method using a hotisostatic pressing furnace were adopted and conducted.

On the sintered body fired by an atmospheric firing method or a pressurefiring method, the tertiary sintering step with the hot press method orthe HIP method applied thereto was further conducted so as to improvethe density of the sintered body.

In the hot press method, a sintered body is pressed from one axialdirection in the sintering process when using a uniaxial hot pressfurnace. With such pressing, compacting of the sintered body can befurther promoted, compared with the atmospheric firing method.

In the HIP method, a sintered body can be pressed from three axialdirections in the sintering process. With such pressing, compacting ofthe sintered body can be further promoted, compared with the atmosphericfiring method.

In order to achieve the above object, a MgF₂ system fluoride sinteredbody for a radiation moderator according to a first aspect of thepresent invention is characterized by comprising MgF₂ having a compactpolycrystalline structure with a bulk density of 3.07 g/cm³ (a relativedensity of 97.5%) or more.

Using the MgF₂ system fluoride sintered body for a radiation moderatoraccording to the first aspect of the present invention, a sintered bodyhaving excellent moderation performance as a neutron moderator andenhanced mechanical strength, which satisfies all the characteristicsrequired for a neutron moderator, can be obtained.

The MgF₂ system fluoride sintered body for a radiation moderatoraccording to a second aspect of the present invention is characterizedby having a bending strength of 12 MPa or more and a Vickers hardness of100 or more as regards mechanical strengths in the MgF₂ system fluoridesintered body for a radiation moderator according to the first aspect ofthe present invention.

Using the MgF₂ system fluoride sintered body for a radiation moderatoraccording to the second aspect of the present invention, a sintered bodyhaving more excellent moderation performance as a neutron moderator andextremely excellent mechanical strength, can be provided.

A method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to a first aspect of the present inventionis characterized by comprising the steps of:

pulverizing a high-purity MgF₂ raw material to control the particlesize, so as to allow the maximum particle diameter in a particle sizedistribution to be 50 μm or less, the shape of the particle sizedistribution curve to be of sub-1-peak type or 1-peak type and themedian diameter to be 6 μm or less;

adding 0.02-1% by weight of a sintering aid to theparticle-size-controlled raw material to mix;

molding the compound at a molding pressure of 5 MPa or more using apress molding device;

molding the press molded article at a molding pressure of 5 MPa or moreusing a cold isostatic pressing (CIP) device;

conducting preliminary sintering by heating the CIP molded article in atemperature range of 600° C.-700° C. in an air atmosphere (preliminarysintering step);

conducting atmospheric sintering or pressure sintering by heating thepreliminary sintered body in a temperature range from (Tn−100)° C. to(Tn)° C. when the starting temperature of foaming of the preliminarysintered body is (Tn)° C., in an air atmosphere or in an inert gasatmosphere or in a vacuum atmosphere (primary sintering step); and

forming a sintered body having a compact structure by heating the samein a temperature range of 900° C.-1150° C. under atmospheric pressure orunder pressure in the same atmosphere as the preceding step (secondarysintering step).

Using the method for producing a MgF₂ system fluoride sintered body fora radiation moderator according to the first aspect of the presentinvention, by pulverizing a raw material and controlling the particlesize thereof, it becomes possible to allow the sintering reaction withlow-temperature heating to easily make progress and to make the degreeof progress of sintering in every part of the sintered body more uniformso as to make it hard to cause a difference in density in the wholesintered body. As a result, a high-density sintered body can beobtained.

Moreover, the difference between parts of the organizational structureof the sintered body is small, and the quantity of generated melt isheld down. The crystal growth of a solid solution is suppressed, leadingto decreased frequency of occurrence of brittle portions. As a result,the strength of the sintered body can be enhanced.

Therefore, the sintered body fired by this method has strong cohesionbetween particles, and high micro strength of the bonding part. Themechanical strength which was a problem to be solved is remarkablyimproved, and the sintered body can be used as a member for a neutronmoderator without problems for actual use.

The crystalline structure of the sintered body fired by this method ispolycrystalline, resulting in remarkable improvement of the brittlenesscompared with a single crystal.

And the highest attained relative density thereof within a range of goodsintering conditions can be increased, and by making wider thetemperature limits of the secondary sintering temperature thereof, inwhich the bulk density thereof becomes high, the stable sinteringconditions can be easily realized.

In the case of pressure sintering, compared with atmospheric sintering,the sintered body has stronger cohesion between particles and highermicro strength of the bonding part, resulting in remarkable improvementof the mechanical strength.

Particularly with the improvement of the particle size condition and thepressure application in the heating process, the degree of compactnessof the sintered body can be noticeably improved.

The method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to a second aspect of the presentinvention is characterized by further comprising the tertiary sinteringstep of reheating in a temperature range of 900° C.-1150° C. underpressure in an inert gas atmosphere or in a vacuum atmosphere in themethod for producing a MgF₂ system fluoride sintered body for aradiation moderator according to the first aspect of the presentinvention.

Using the method for producing a MgF₂ system fluoride sintered body fora radiation moderator according to the second aspect of the presentinvention, it becomes easy to defoam in the sintering process and itbecomes far easier to enhance the relative density of the sintered body.The sintered body can be allowed to have more excellent moderationperformance as a neutron moderator, a density remarkably improvedcompared with the sintered body on which only primary and secondarysintering were conducted, and mechanical strength remarkably enhanced.

A MgF₂ system fluoride sintered body for a radiation moderator accordingto a third aspect of the present invention is characterized by being aMgF₂—CaF₂ binary system fluoride sintered body having a compactpolycrystalline structure, containing CaF₂ of 90% by weight at themaximum with a relative density of 95.2% or more.

Using the MgF₂ system fluoride sintered body for a radiation moderatoraccording to the third aspect of the present invention, the differencebetween parts of the organizational structure of the sintered body issmall, and the quantity of generated melt is held down. The crystalgrowth of a solid solution is suppressed, leading to decreased frequencyof occurrence of brittle portions. As a result, the strength of thesintered body can be enhanced. Therefore, a sintered body havingexcellent moderation performance as a neutron moderator and enhancedmechanical strength, which satisfies all the characteristics requiredfor a neutron moderator, can be obtained.

The MgF₂ system fluoride sintered body for a radiation moderatoraccording to a fourth aspect of the present invention is characterizedby having a bending strength of 13 MPa or more and a Vickers hardness of100 or more as regards mechanical strengths in the MgF₂ system fluoridesintered body for a radiation moderator according to the third aspect ofthe present invention.

Using the MgF₂ system fluoride sintered body for a radiation moderatoraccording to the fourth aspect of the present invention, a sintered bodyhaving more excellent moderation performance as a neutron moderator andextremely excellent mechanical strength can be provided.

A method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to a third aspect of the present inventionis characterized by comprising the steps of:

mixing a high-purity MgF₂ powder with a high-purity CaF₂ powder in sucha manner that the content of CaF₂ is 90% by weight at the maximum, andthen pulverizing the same to control the particle size, so as to allowthe maximum particle diameter in a particle size distribution to be 50μm or less, the shape of the particle size distribution curve to be ofsub-1-peak type or 1-peak type and the median diameter to be 6 μm orless;

adding 0.02-1% by weight of a sintering aid to theparticle-size-controlled raw material to mix;

molding the compound at a molding pressure of 5 MPa or more using apress molding device;

molding the press molded article at a molding pressure of 5 MPa or moreusing a cold isostatic pressing (CIP) device;

conducting preliminary sintering by heating the CIP molded article in atemperature range of 600° C.-700° C. in an air atmosphere (preliminarysintering step);

conducting atmospheric sintering or pressure sintering by heating thepreliminary sintered body in a temperature range from (Tn−100)° C. to(Tn)° C. when the starting temperature of foaming of the preliminarysintered body is (Tn)° C., in an air atmosphere or in an inert gasatmosphere or in a vacuum atmosphere (primary sintering step); and

forming a sintered body having a compact structure by heating the samein a temperature range of 900° C.-1150° C. under atmospheric pressure orunder pressure in the same atmosphere as the preceding step (secondarysintering step).

Using the method for producing a MgF₂ system fluoride sintered body fora radiation moderator according to the third aspect of the presentinvention, the sintered body fired by this method has strong cohesionbetween particles, and high micro strength of the bonding part. Themechanical strength which was a problem to be solved is remarkablyimproved, and the sintered body can be used as a member for a neutronmoderator without problems for actual use.

The crystalline structure of the sintered body fired by this method ispolycrystalline, resulting in remarkable improvement of the brittlenesscompared with a single crystal.

And the highest attained relative density thereof within a range of goodsintering conditions can be increased, and by making wider thetemperature limits of the secondary sintering temperature thereof, inwhich the bulk density thereof becomes high, the stable sinteringconditions can be easily realized.

The method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to a fourth aspect of the presentinvention is characterized by further comprising the tertiary sinteringstep of reheating in a temperature range of 900° C.-1150° C. underpressure in an inert gas atmosphere or in a vacuum atmosphere in themethod for producing a MgF₂ system fluoride sintered body for aradiation moderator according to the third aspect of the presentinvention.

Using the method for producing a MgF₂ system fluoride sintered body fora radiation moderator according to the fourth aspect of the presentinvention, it becomes easy to defoam in the sintering process and itbecomes far easier to enhance the relative density of the sintered body.A sintered body having more excellent moderation performance as aneutron moderator and extremely excellent mechanical strength can beprovided.

The method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to a fifth aspect of the present inventionis characterized by the inert gas atmosphere in the two sintering steps(primary and secondary sintering steps) comprising one kind of gas or amixture of plural kinds of gases, selected from among nitrogen, heliumand argon, wherein using a hot press furnace or a hot isostatic pressingfurnace, hot molding work is conducted in the heating process in themethod for producing a MgF₂ system fluoride sintered body for aradiation moderator according to the first or third aspect of thepresent invention.

Using the method for producing a MgF₂ system fluoride sintered body fora radiation moderator according to the fifth aspect of the presentinvention, it is possible to make it far easier to defoam in thesintering process, and to make it far easier to increase the relativedensity of the sintered body.

The method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to a sixth aspect of the present inventionis characterized by the inert gas atmosphere in the three sinteringsteps (primary, secondary and tertiary sintering steps) comprising onekind of gas or a mixture of plural kinds of gases, selected from amongnitrogen, helium and argon, wherein using a hot press furnace or a hotisostatic pressting furnace, hot molding work is conducted in theheating process in the method for producing a MgF₂ system fluoridesintered body for a radiation moderator according to the second orfourth aspect of the present invention.

Using the method for producing a MgF₂ system fluoride sintered body fora radiation moderator according to the sixth aspect of the presentinvention, it is possible to make it far easier to defoam in thesintering process, and to make it far easier to increase the relativedensity of the sintered body.

The method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to a seventh aspect of the presentinvention is characterized by in the two sintering steps (primary andsecondary sintering steps), in a vacuum atmosphere of less than 100 Pa,conducting hot molding work using a hot press furnace in the heatingprocess in the method for producing a MgF₂ system fluoride sintered bodyfor a radiation moderator according to the first or third aspect of thepresent invention.

Using the method for producing a MgF₂ system fluoride sintered body fora radiation moderator according to the seventh aspect of the presentinvention, it is possible to make it far easier to defoam in thesintering process, and to make it far easier to increase the relativedensity of the sintered body.

The method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to an eighth aspect of the presentinvention is characterized by in the three sintering steps (primary,secondary and tertiary sintering steps), in a vacuum atmosphere of lessthan 100 Pa, conducting hot molding work using a hot press furnace inthe heating process in the method for producing a MgF₂ system fluoridesintered body for a radiation moderator according to the second orfourth aspect of the present invention.

Using the method for producing a MgF₂ system fluoride sintered body fora radiation moderator according to the eighth aspect of the presentinvention, it is possible to make it far easier to defoam in thesintering process, and to make it far easier to increase the relativedensity of the sintered body.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of a particle size distributionwhen a raw material of MgF₂ simple was pulverized;

FIG. 2 is a block flow diagram showing the steps of producing a MgF₂system fluoride sintered body;

FIG. 3 is a phase diagram of the MgF₂—CaF₂ binary system;

FIG. 4 is a diagram showing the relationship between the secondarysintering temperatures and the relative densities of sintered bodies ofMgF₂ simple with varied particle size conditions of the compound;

FIG. 5 is a diagram showing the relationship between the secondarysintering temperatures and the relative densities of MgF₂—CaF₂ binarysystem sintered bodies with varied particle size conditions of thecompound;

FIG. 6 is a diagram showing the relationship between the relativedensities of the sintered bodies produced by conducting primary andsecondary sintering on a MgF₂ simple raw material using three kinds offiring furnaces and the secondary sintering temperatures;

FIG. 7 is a diagram showing the relationship between the relativedensities of the sintered bodies produced by conducting primary andsecondary sintering on a MgF₂—CaF₂ compound using three kinds of firingfurnaces and the secondary sintering temperatures;

FIG. 8 comprises SEM photographs showing an original raw material andthe raw material after pulverized for three weeks;

FIG. 9 is Table 1 showing the measurement results of the neutronmoderation performance of MgF₂—CaF₂ binary system sintered bodies havingvaried raw material mix proportions;

FIG. 10 is Table 2 showing measured data of Examples; and

FIG. 11 is Table 3 showing measured data of Comparative Examples.

DESCRIPTION OF EMBODIMENTS

The MgF₂ system fluoride sintered body (including a MgF₂ simple sinteredbody and a MgF₂—CaF₂ binary system sintered body, hereinafter, referredto similarly) for a radiation moderator having a compact polycrystallinestructure excellent in radiation moderation performance, especiallyneutron moderation performance according to the preferred embodiments ofthe present invention and the producing method thereof are describedbelow by reference to the Figures.

The method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to the preferred embodiment of the presentinvention, as shown in FIG. 2, comprises the steps of raw materialmixing, particle size controlling, uniaxial press molding, CIP molding,preliminary sintering, primary sintering and secondary sintering.

A great difference between a method for producing a MgF₂ simple sinteredbody and that for producing a MgF₂—CaF₂ binary system sintered body isthat the method for producing a MgF₂ simple sintered body does notrequire the “raw material mixing” step. After the raw material mixingstep, in both methods, every step may be advanced almost in the samemanner.

In the method for producing a MgF₂ system fluoride sintered body for aradiation moderator according to the preferred embodiment, a high-purity(purity of 99.9% by weight or more) MgF₂ raw material was mixed with ahigh-purity (purity of 99.9% by weight or more) CaF₂ raw material in theproportion of 0-90% by weight (included in a total of 100). (Here, 0% byweight means a case where a CaF₂ raw material is not mixed, that is, aMgF₂ simple raw material. In producing a MgF₂ simple sintered body, the“raw material mixing” step is not required as mentioned above.) Mixingof the raw materials was conducted by mixing for 12 hours using a Vmixer.

Then, particle size control of the raw material powder was conducted.

As the particle size control, using a ball mill (a container of a potmill made of alumina: an inside diameter of 200 mm and a length of 250mm), balls made of alumina, ϕ20 mm:3000 g and ϕ30 mm:2800 g, and about3000 g of the raw material powder were filled therein and thereafter,rotated for a prescribed period of time. The rotation was stopped everytwo or three days so as to take sample powders and measure the same.

The particle size distribution were measured using ‘a laser diffractionparticle size analyzer (type number: SALD-2000)’ made by ShimadzuCorporation according to JIS R1629 ‘Determination of particle sizedistributions for fine ceramic raw powders by laser diffraction method’.The sample preparation at that time was conducted according to JIS R1622‘General rules for the sample preparation of particle size analysis offine ceramic raw powder’.

As the light source of the SALD-2000, a semiconductor laser of awavelength of 680 nm is used. The sensitivity to particles having adiameter larger than this wavelength (about 1 μm or more) was good andthe measurement accuracy was high. On the other hand, as for thesensitivity to fine particles of the order of submicron, it wasconsidered that the measurement accuracy was low compared with theparticles having a large diameter though some way to improve themeasurement accuracy was devised.

Therefore, it is considered that the actual number of fine particles ofthe order of submicron may be larger than the analysis result. In otherwords, ‘there is a high possibility that the ratio of fine particles maybe larger than the analysis result in the actual particle sizedistribution, and that the mean particle size may be smaller than theshown value thereof.’ However, in the present application, the values ofthe particle sizes measured according to the above measurement methodare shown as they are.

FIG. 1 shows a particle size distribution in the case of the aboveparticle size control (pulverization) of a MgF₂ raw material. It wasproved that the median diameter could be about 10 μm after three-daypulverization, about 8 μm after five-day pulverization, about 6 μm afterone-week pulverization, about 5 μm after two-week pulverization, about 4μm after three-week pulverization, and about 3 μm after four-weekpulverization. Even if a MgF₂ raw material and a CaF₂ raw material weremixed, the particle size distribution similar to that of the rawmaterial of MgF₂ simple could be obtained.

SEM photographs of particles of the original raw material powder and theabove particles whose median diameter became about 4 μm after three-weekpulverization are shown in FIG. 8. In the particles of the original rawmaterial powder, some irregular-shaped particles, mainly angularparticles were seen, while most of the particles after three-weekpulverization were rounded. It was found that most of the angularportions of the particles of the original raw material powder were wornby pulverization so as to be approximated to sphere shapes.

Judging from the particle size distribution curve of the powder afterparticle size control, the curve of three-day pulverization and that offive-day pulverization obviously showed a high ratio of coarse particleportions, respectively. When the shape of the curve looks like as if“two peaks” or “three peaks” run in a line, which is called ‘2-peaktype’ or ‘3-peak type’ by being likened to the shape of a mountainrange, they were regarded as ‘2-peak type’ or ‘3-peak type’.

In the case of one-week pulverization and two-week pulverization,respectively, the ratio of the coarse particle portions substantiallydecreased, the coarse particle portion of 30 μm or more remained several% by weight, but that of 50 μm or more almost disappeared, and the shapeof the mountain range was reaching almost the 1-peak type having a smallgently inclined portion around the particle diameter of 10 μm-15 μm(this state is called ‘sub-1-peak type’). And it was found that in thecase of three-week or more pulverization, the coarse particle portion of30 μm or more almost disappeared and that the shape of the particle sizedistribution curve could be approximately similar to a normaldistribution (this state is called ‘1-peak type’. In this state, themean diameter and the median diameter are about to be equal.).

Thus, the particle shapes were rounded and approached sphere shapes bypulverization of the raw material, and the ratio of coarse particlesdecreased, resulting in a great change in shape of the particle sizedistribution curve from ‘2-peak type’ or ‘3-peak type’ to ‘sub-1-peaktype’, and further to ‘1-peak type’. This change exerted a noticeableinfluence on sintering reaction in the sintering process, which isdescribed below with comparative examinations (the results of thecomparative examinations are shown in FIGS. 4 and 5).

Furthermore, as a sintering aid, a carboxymethyl cellulose (CMC)solution was added thereto in the proportion of 0.02-1% by weight (notincluded in 100) to 100 of the mixture and mixed. The mixture was usedas a starting raw material (raw material mixing step).

In the uniaxial press molding step, using a wooden mold form, thestarting raw material of a prescribed quantity is filled in the moldform and thereafter, a prescribed molding pressure of 5 MPa or more isuniaxially applied thereto using a press jig for molding (a methodwherein by moving the press jig downward from the upper side whileapplying pressure, the starting raw material in the mold form iscompressed). By this press molding, without causing deformation in thehandling process of the press molded body such as taking from the moldform for molding, it is possible to secure the hardness of the moldedbody for keeping the form thereof.

In the CIP molding step, this press molded body is molded at a moldingpressure of 5 MPa or more using a cold isostatic pressing (CIP) deviceso as to form a CIP molded body.

In this step of molding using the CIP device, the press molded bodyafter uniaxial press molding is put into a thick vinyl bag, which isthen deaired and sealed, and it is put into the molding part of the CIPdevice having a structure wherein the device body can be split to theupper and lower portions, which is then sealed. The space between thevinyl bag with the press molded body therein and the sealed molding partis filled with clean water, and then, isostatic pressing is conducted ata hydraulic pressure of 5 MPa or more, which is the CIP molding.

Through this CIP molding, it is possible to conduct compression on thepress molded body which is a uniaxially compressed molded body from allthe directions of three axes so as to make the spaces between the rawmaterial particles smaller in all directions. Accordingly, sinteringreaction in the following preliminary, primary and secondary sinteringsteps, or preliminary, primary, secondary and tertiary sintering stepscan be promoted.

Preliminary sintering is conducted by heating this CIP molded body in atemperature range of 600° C.-700° C. in an air atmosphere (preliminarysintering step).

In the primary sintering step, the preliminary sintered body sintered inthe preliminary sintering step is sintered in a temperature range justbelow the starting temperature of foaming Tn:

(a) using an atmospheric firing furnace in which the atmosphere can beadjusted, in an air or in an inert gas atmosphere, for example, in anitrogen gas atmosphere;

(b) using a uniaxial pressing hot press furnace, by a hot press methodin an inert gas atmosphere or in a vacuum atmosphere; or

(c) using a hot isostatic pressing furnace, by a hot isostatic pressing(HIP) method in a pressurized inert gas atmosphere.

The sintering temperature in the primary sintering step was set to rangefrom (Tn−100° C.) to Tn when the starting temperature of foaming was Tn.As described above, the starting temperature of foaming Tn wasdetermined from the measured temperatures using a differential thermalanalyzer, and the temperature varied in a range from about 850° C. to900° C. depending on the mix proportion of the raw materials of MgF₂ andCaF₂.

Specifically, the temperature range of 750° C.-850° C. in the case of acomposition mainly comprising MgF₂, that of 800° C.-900° C. in the caseof a composition mainly comprising CaF₂, and that of 775° C.-875° C. inthe case of an intermediate composition of the both, were determineddepending on the composition.

In the primary sintering step, by heating in these temperature rangesfor a relatively long period of time (4-16 hours), sintering is allowedto make progress uniformly all over the sintered body.

In the secondary sintering step, by heating in the same furnace and thesame atmosphere as the primary sintering step, in a temperature range of900° C.-1150° C. in the vicinity of the temperature limits in which asolid solution starts to be formed (the temperature limits in thevicinity of 980° C., being a temperature at which a solid solutionstarts to be formed in the MgF₂—CaF₂ binary system phase diagram in FIG.3) for a relatively short period of time (0.5-8 hours), a sintered bodyhaving a compact structure is formed.

‘A solid solution’ here is a state of a solid in single phase in whichcomponents of a plurality of compounds having similar crystal state, forexample, those of MgF₂ and CaF₂ in the case of this sintered body, areuniformly and indiscriminately distributed in a crystal.

In some cases, after conducting the above primary and secondarysintering steps, by the hot press method or the HIP method, another stepof firing with the primary or secondary sintering conditions, the samewith the case of the atmospheric firing furnace so as to form a MgF₂system fluoride sintered body having a more compact structure (tertiarysintering step) may be conducted.

Here, as an inert gas, other than nitrogen (N₂), helium (He) or neon(Ne), argon (Ar) or the like is used.

The reason why the sintering step was divided into two steps, primaryand secondary, is in order to suppress foaming as much as possible, andmake the difference of the degree of sintering progress in every part(such as a periphery portion and a center portion) of the sintered bodyas small as possible.

Particularly, in order to produce a large-size compact sintered body,the technique is important. The large size here is applied to pressmolded bodies in the below-described Examples having the size of about220 mm×220 mm×H85 mm, while the small size is applied to thebelow-mentioned press molded bodies having the size of dia. 80 mm×H50mm.

In a test conducted in order to roughly grab proper heating conditionsof the sintering step, the starting raw materials (three kinds ofparticle sizes were selected) comprising MgF₂ simple and MgF₂—CaF₂binary system, respectively, were used, the sample size was the abovelarge size, and both of the two stages of sintering were conducted in anitrogen gas atmosphere. In the primary sintering, the temperature washeld at 840° C. for 6 hours and in the subsequent secondary sintering,the heating time was set to be 2 hours with varied heating temperaturesso as to measure the relative densities of the sintered bodies.

As a result, as shown in FIGS. 4 and 5, the relative densities of 95% ormore could be secured in a wide range of heating conditions whentwo-stage sintering step was conducted, in either case of MgF₂ simpleand MgF₂—CaF₂ binary system, even in the test on large-size samples.

Particularly, in the case of MgF₂—CaF₂ binary system, with the threekinds of particle sizes of the raw material shown in the figure, therelative densities of 96%-97% could be obtained in the good conditionrange (heating at 1025° C.-1075° C.).

On the other hand, it is not shown in the figure, but when onlyone-stage sintering step was conducted on the samples of large size asin the case of the two-stage sintering, the relative densities were 94%or less.

The aim of mixing a CaF₂ powder being a secondary raw material into aMgF₂ powder being a main raw material to form MgF₂—CaF₂ binary system isto cause the sintering reaction which allows the region of the formationof a solid solution on the phase diagram shown in FIG. 3 to becomeclearer, since MgF₂ simple has a high melting point of 1252° C. and thetemperature region of the formation of a solid solution is partiallyunclear, shown with dot lines.

By mixing the right quantity of CaF₂, being a fluoride of Ca which ispresumed to have similar characteristics to Mg since Ca belongs to thesame group as Mg on the periodic table of elements and its period isnext to Mg, the melting point can be lowered and the temperatureconditions of the formation of a solid solution can be clarified.

By mixing CaF₂, the melting point can be moved from the dot line regionon the left end portion of the line indicating the temperature region ofstarting of the formation of a solid solution in FIG. 3 toward the solidline region of the intermediate mix proportions positioned on the righthand. As a result, it becomes easy to make the sintering temperatureconditions proper.

As a material to be mixed into MgF₂ other than CaF₂ being a fluoride ofCa, LiF being a fluoride of Li can be exemplified.

The mix proportion of MgF₂ being a main raw material and CaF₂ being asecondary raw material were varied in a range of 0-97.5% by weight(included in the total). The raw material and balls were filled into theball mill, which was rotated for a week so as to make the mediandiameter about 6 μm. The ball mill made of alumina having an insidediameter of 280 mm and a length of 400 mm was used, and balls of ϕ5mm:1800 g, ϕ10 mm:1700 g, ϕ20 mm:3000 g and ϕ30 mm:2800 g, made ofalumina were filled therein.

As the sintering aid added to the particle-size-controlled raw materialof 3000 g, two kinds, the CMC and the calcium stearate (SAC), wereselected. With various adding proportions of each of them in theproportion of 0-2% by weight (not included in the total), the effects ofaddition thereof were confirmed. For comparison, a test with nosintering aid was also conducted. After adding the sintering aid, theraw material was mixed using a V mixer for 12 hours to be a compound.

This compound of a prescribed quantity was filled into a wooden moldform, and using a uniaxial press device, compressed and molded at auniaxial press pressure of 5 MPa or more. The inside size of the moldform for a large-size test was 220 mm×220 mm×H150 mm, and the insidesize of the mold form for a small-size test was 80 mm in diameter and100 mm in height.

This press molded body was put into a thick vinyl bag, which was thendeaired and sealed, and it was put through a cold isostatic pressing(CIP) device. The press molded body was put into the molding part havinga two-split structure (inside diameter 350 mm×H120 mm), which wassealed. The space between the vinyl bag with the press molded bodytherein and the molding part was filled with clean water, and then,isostatic pressing was conducted at a hydraulic pressure of 5 MPa ormore so as to form a CIP molded body.

The preliminary sintering step was conducted on the CIP molded bodies inan air atmosphere with various kinds of conditions in a heatingtemperature range of 500° C. to 750° C. and in a heating time range of 3to 18 hours.

After observing the appearance of the preliminary sintered bodies, thepreliminary sintered bodies were sintered with the conditions which wereregarded as good sintering conditions in the preceding preliminary test.The sintering step was conducted with the conditions wherein, in anitrogen gas atmosphere, the temperature was raised from roomtemperature to 600° C. at a fixed rate for 6 hours, and held there for 8hours, and then, it was raised to 1000° C. at a fixed rate for 2 hoursand held there for 1 hour. And thereafter, it was lowered to 100° C. for20 hours.

By observing the appearance of the taken-out sintered bodies, the stateof compactness of the inside thereof and the like, proper raw materialmix proportions, raw material processing conditions, preliminarysintering conditions and the like were investigated.

As a result, in cases where the mix proportion of the secondary rawmaterial CaF₂ to the main raw material MgF₂ was 90.1% by weight or more,a larger number of large bubbles were left in the inside portion of thesintered body, compared with the periphery portion thereof, resulting ininsufficient compactness. It was presumed that the reason was becausethe sintering speed was too fast.

Judging from these situations, the mix proportions of CaF₂ to MgF₂, inwhich the difference in compactness between the inside portion and theperiphery portion of the sintered body was small, that is, the sinteringperformance was in a good state, were 0-90% by weight. It was confirmedthat the more desirable mix proportions thereof in which the differencein compactness between the inside portion and the periphery portion ofthe sintered body was smaller, resulting in an excellent degree ofuniformity, were 0-50% by weight. Hence, the proper range of mixproportions of CaF₂ was judged to be 0-90% by weight, more desirably0-50% by weight.

There was no big difference between the effects of the two kinds ofsintering aids, but when the mix proportion of the sintering aid wasless than 0.02% by weight, the shape keeping performance of the moldedbody was poor. And when the mix proportion thereof exceeded 1.1% byweight, coloring which appeared to be a residual of the sintering aidwas noticed on the preliminary sintered body or the sintered body insome cases. Hence, the proper range of mix proportions of the sinteringaid was judged to be 0.02-1% by weight.

In a uniaxial press test using the above wooden mold form for asmall-size test, when the molding pressure of the uniaxial press devicewas less than 5 MPa, the press molded body easily lost its shape inhandling. As the molding pressure was gradually increased from 5 MPa,the bulk density of the press molded body gradually increased, and itwas recognized that the bulk densities of the preliminary sintered bodyand the sintered body also tended to increase though slightly. The testwas conducted with the molding pressure gradually increased to 100 MPa.However, even if the molding pressure was raised to 20 MPa or more, noimprovement of performance of the preliminary sintered body or thesintered body was recognized. Hence, the proper value of the moldingpressure of the uniaxial press device was decided to be 5 MPa or more,desirably 20 MPa.

When the molding pressure of the CIP device was less than 5 MPa, the CIPmolded body easily lost its shape in handling. As the molding pressurewas gradually increased from 5 MPa, the bulk density of the CIP moldedbody gradually increased, and it was recognized that the bulk densitiesof the preliminary sintered body and the sintered body also tended toincrease though slightly. The test was conducted with the CIP moldingpressure gradually increased to 60 MPa. However, even if the moldingpressure was raised to 20 MPa or more, no great improvement ofperformance of the preliminary sintered body or the sintered body wasrecognized. Hence, the proper value of the molding pressure of the CIPdevice was decided to be 5 MPa or more, desirably 20 MPa.

The research of preliminary sintering conditions of the CIP molded bodyin an air atmosphere was conducted under the below-described conditions.By mixing MgF₂ with CaF₂ of 3% by weight, and adding CMC of 0.1% byweight as a sintering aid thereto, a starting raw material was prepared.Using the wooden mold form for a small-size test, by setting the moldingpressure of a uniaxial press device to be 20 MPa and setting the moldingpressure of a CIP device to be 20 MPa, CIP molded bodies were formed.Using the CIP molded bodies formed under such conditions, thepreliminary sintering conditions were researched.

As a result of the research, at heating temperatures of less than 600°C., shrinkage was small compared with the size of the molded body, whileat heating temperatures of 710° C. or more, the shrinkage rate was toohigh and therefore, shrinkage was difficult to control. Hence, theproper range of the preliminary sintering temperatures was decided to be600° C.-700° C.

Concerning the heating time, at 600° C., it was judged that 8-9 hourswere optimal, and that 4-10 hours were proper, judging from theevaluation of the shrinkage rate. At 700° C., it was judged that 6-8hours were optimal, and that 4-10 hours were proper. From these results,the heating conditions in the preliminary sintering step were decided tobe at 600° C.-700° C. for 4-10 hours in an air atmosphere.

What is likely to give most influence on the performance of a sinteredbody in producing the MgF₂ system fluoride sintered body for a radiationmoderator is the sintering step. From the above researches and tests,the proper conditions until just before the sintering step wereclarified.

Before clarifying the proper conditions in the sintering step, thesintering step and the sintering mechanism which appear to be desirableto a MgF₂ system fluoride sintered body for a radiation moderator areput in order.

The terms “primary flocculation process” and “secondary flocculationprocess” which express the degrees of progress of the sintering step,are described below. The “primary flocculation process” refers to anevent in the first half of the stage of sintering, and in the initialstage thereof, the intervals between particles gradually become narrowerand the voids among particles also become smaller. With further progressof sintering, the particle-to-particle contact portions become thick andthe voids among them become further smaller. Here, the majority of thevoids are open pores connecting to the surrounding atmosphere. Up tothis stage is called “primary flocculation process”.

On the other hand, after the end of the primary flocculation process,with further progress of sintering, the open pores gradually decreaseand turn into closed pores. The stage of turning into closed pores andthe subsequent stage of defoaming and compacting are generically called“secondary flocculation process”.

In the producing method according to the preferred embodiment, due toraw material mixing, particle size control, sintering aid adding/mixing,two-stage molding (uniaxial press molding and CIP molding), preliminarysintering and the like, it was noticed that the voids among particles ofthe preliminary sintered body were small, and that the voids almostuniformly scattered without gathering (the first half stage of theprimary flocculation process).

In the heating process of the next sintering step (corresponding to theprimary sintering step), the heating temperature is gradually raised.Around the temperature limits (500° C.-550° C.) slightly lower than thepreliminary sintering temperatures (600° C.-700° C.), particles start togather, and thereafter, solid phase reaction starts in the temperaturelimits far lower than 980° C. at which a solid solution starts to beformed. With that, flocculation of particles makes progress, leading toshorter particle-to-particle distances and smaller voids.

It is generally said that the solid phase reaction starts in thetemperature limits lower by the order of 10% or further lower than thetemperature at which a solid solution starts to be formed. From theobservation results in the preliminary test by the present inventors, itwas considered that the solid phase reaction started in far lowertemperature limits than the above generally said temperature limits, inthe order of 500° C.-550° C.

It can be said on the ground that at 600° C., the lowest limit of thepreliminary sintering temperature, sintering by the solid phase reactionhas already made progress considerably so that the preliminary sinteredbody considerably shrinks compared with the CIP molded body.

It is considered that the solid phase reaction makes progress at a lowreaction rate in the temperature limits and that it makes progress at aquite high reaction rate in the temperature limits in the vicinity of750° C., or more up to 980° C. Here, in the case of heating atrelatively low temperatures (600° C.-700° C.) like assumed preliminarysintering for a short period of time, most of the voids remain in astate of open pore (which is the first half stage of the primaryflocculation process).

What attention should be paid to here are the solid phase reactionvarying depending on the mix proportion of the raw material and behaviorof fine bubbles (foaming bubbles) generated through vaporization of partof the raw material in the temperature limits of about 850° C.-900° C.or more, as mentioned above. In the case of heating at about 1000° C. ormore, the heating time should be as short as possible, since thisformation of foaming bubbles comes to be noticeable.

In the producing method according to the preferred embodiment, thesintering step is divided into two. In the primary sintering step, byheating in the relatively low temperature limits in which no foamingbubbles are formed for a long period of time, sintering of the wholebody is allowed to make progress almost uniformly. The micro structureof the sintered body comprises mainly open pores, but part of them isturned into closed pores (after finishing the second half stage of theprimary flocculation process, partially in the secondary flocculationprocess).

In the secondary sintering step, heating is conducted in the relativelyhigh temperature limits in the vicinity of 980° C. at which a solidsolution starts to be formed for a minimum required period of time. Asthe micro structure of the sintered body, the formation of foamingbubbles is suppressed as much as possible, while the sintering reactionis allowed to make progress so as to turn almost all the open pores intoclosed pores, that is, the secondary flocculation process is finished,resulting in obtaining a high-density sintered body.

Micro behavior of raw material particles is described here. As the microbehavior of raw material particles in the sintering step, in the case ofa raw material of MgF₂ simple, it is considered that the solid phasereaction starts from the portion at which particles make contact witheach other, and that the particle-to-particle contact area graduallyincreases.

On the other hand, in the case of a raw material of MgF₂—CaF₂ binarysystem, it is presumed that particles of CaF₂ being a secondary rawmaterial are present around particles of MgF₂ being a main raw materialand promote interface reaction with the particles of MgF₂ at a higherreaction speed than the case of MgF₂ simple.

Around a heating temperature exceeding 980° C. at which a solid solutionstarts to be formed, the reaction starts in the vicinity of a particleinterface where the particles of CaF₂ are present around the particlesof MgF₂, and a solid solution of a MgF₂—CaF₂ binary system compoundstarts to be formed. It is presumed that this solid solution fills thevoids among particles and that in some part, finer voids are also filledtherewith through capillary phenomenon.

On the other hand, even if the heating temperature is lower than 980°C., by heating and holding at about 750° C. or more for a relativelylong period of time as described above, the solid phase reaction easilymakes progress, the voids gradually decrease with the elapse of time soas to be closed pores. Parallel with that, a gas component within theclosed pores diffuses within the bulk (parent) of the sintered body,leading to the progress of defoaming so as to make the sintered bodycompact with few bubbles (this state is the secondary flocculationprocess).

Also here, in heating at temperatures not lower than the startingtemperature of foaming Tn (as described above, the starting temperatureof foaming differs depending on the mix proportion of the raw materialsMgF₂ and CaF₂), that is, temperatures exceeding 850° C.-900° C.,attention should be paid to the formation of fine bubbles (foamingbubbles) generated through vaporization of the raw material.

That is because it is presumed that the foaming bubbles contain fluorinegas, and it is considered that this gas is a relatively heavy elementand difficult to diffuse in the bulk of the sintered body. As measuresfor that, to avoid heating in the temperature limits of vaporization asmuch as possible, and if necessary, to heat at a temperature as low aspossible or to heat for a short period of time are considered.

The difference in appearance between such foaming bubbles and bubblesleft after pores became closed but could not be defoamed in thesintering step (hereinafter, referred to as residual bubbles) isdescribed below. The sizes of the foaming bubbles generated by heatingfor a relatively short period of time are approximately several μm indiameter, and the shapes thereof are almost perfect spheres.

On the other hand, the shapes of the residual bubbles are not perfectspheres but irregular, and the sizes thereof are all mixed up, large,medium and small. Therefore, it is possible to distinguish the bothaccording to the difference in shape. Here, in the case ofhigh-temperature heating at temperatures far exceeding 1160° C., orheating at temperatures exceeding 1160° C. for a long period of time, afoaming bubble and a foaming bubble, or a residual bubble and a foamingbubble gather and grow to a large irregular bubble in some cases,resulting in difficulty in judging its origin.

With the progress of the secondary flocculation process, the voids amongparticles become smaller, and all or most of the voids are surrounded byparticles or a bridge portion of the sintered body so as to be closedpores (bubbles). Depending on the conditions, there is another casewherein gases are released through the voids (open pores), or gaseswithin the bubbles permeate into the bulk (parent) such as the particlesor the bridge portion of the sintered body to degas, resulting in nobubbles (referred to as ‘a defoaming phenomenon’ or simply ‘defoaming’).

Whether the voids among particles are left as closed pores, that is,bubbles, or by degassing, no bubbles are formed, is a significantelement for deciding the degree of achievement of compactness of thesintered body, leading to deciding the characteristics of the sinteredbody.

Particularly in the case of sintering in a light element gas atmospheresuch as He or Ne among inert gases, it is considered that the lighterelement more easily diffuses within the pores or the bulk of thesintered body, leading to promoting the capillary phenomenon anddefoaming phenomenon, so that bubbles are difficult to remain, leadingto easy compacting.

Thus, in order to make the whole compact, it is important to advance theprimary flocculation process (in detail, it is presumed that the primaryflocculation process is divided into the first half stage and the secondhalf stage) and the secondary flocculation process almost simultaneouslyand almost uniformly on the whole in each process.

In the producing method according to the preferred embodiment of thepresent invention, the preliminary sintering step chiefly equivalent tothe first half stage of the primary flocculation process, the primarysintering step chiefly equivalent to the second half stage of theprimary flocculation process, and the secondary sintering step chieflyequivalent to the secondary flocculation process are separatelyconducted, so as to make the two flocculation processes easy to makeprogress almost uniformly throughout the sintered body.

However, Even if the sintering step is divided into two steps ofpreliminary sintering and sintering like this, a noticeable differencein degree of compactness is caused without proper heating conditions.For example, in the case of heating at high temperatures exceeding theproper limits in the preliminary sintering step, in the case of rapidlyheating at the temperature raising stage of the sintering step, or incases where the holding temperature in the sintering step is a hightemperature exceeding the proper limits, a remarkable difference indegree of compactness is caused between the periphery portion and theinside portion of the sintered body. By improper heating, degassingbecomes difficult in the process of compacting of the inside portion ofthe sintered body, and the compactness of the inside portion thereof islikely to be insufficient.

It means that it is important to make the heating temperature pattern inthe sintering step proper according to the size. Particularly, whenproducing a large-size sintered body, it is necessary to strictlycontrol the heating conditions since a great difference in degree ofcompactness between the periphery portion and the inside portion of suchsintered body is easily caused.

In order to clarify the relationship between the sample size and thesintering state, the present inventors conducted a small-size test usingsamples molded in a mold form of a uniaxial press device the inside sizeof 80 mm in diameter and 100 mm in height, and a large-size test usingsamples molded in a mold form thereof the inside size of 220 mm×220mm×H150 mm.

As a result, in the small-size test, there were cases where ahigh-density sintered body having a relative density exceeding 95% wasobtained depending on the heating conditions even if one sintering stepwas conducted. On the other hand, in the large-size test, with onesintering step, any of the sintered bodies had a low density of lessthan 94% under the same heating conditions as the small-size test.

What is important here is that the whole of the preliminary sinteredbody has already advanced almost uniformly to the first half stage ofthe primary flocculation. Only preliminary sintered bodies in a state inwhich the whole body has already advanced to the first half stage of theprimary flocculation were provided to these tests of the sintering step.

The Description of the Sintering Step Test

A mixture of a main raw material MgF₂ with CaF₂ of 3% by weight, and araw material of MgF₂ simple were used as starting materials. CMC of 0.1%by weight was added thereto as a sintering aid. And using the above moldform for a large-size test, the compounds were molded at a moldingpressure of 20 MPa of a uniaxial press device and at a molding pressureof 20 MPa of a CIP device.

The CIP molded bodies were preliminary sintered at 650° C. for 6 hoursin an air atmosphere so as to form preliminary sintered bodies.

In a nitrogen gas atmosphere, as primary sintering step, the preliminarysintered bodies were heated to 840° C. and the temperature was heldthere for 6 hours and then raised to a secondary sintering temperaturefor 2 hours.

The secondary sintering temperature was varied from 700° C. to 1250° C.,at an interval of every 50° C., and the temperatures each were held for2 hours.

Thereafter, the heating was stopped and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours, and whenreaching 100° C. or lower at which time it was previously set to takeout the sintered body, it was taken out.

As a result of the sintering test with such two-stage sintering step, inthe case of a sintering temperature range from 900° C. to 1150° C., mostof the bulk densities of the sintered bodies exceeded 2.96 g/cm³, whichwere high. The true density of the binary system compound was 3.15 g/cm³and the relative density thereof was 94.0%, while the true density ofthe raw material of MgF₂ simple was also 3.15 g/cm³ and the relativedensity thereof was also 94.0%.

In either case of sintering temperatures of less than 900° C., and thoseof 1160° C. or more, the relative densities were lower than 94.0% (thebulk density of 2.96 g/cm³). The sintered bodies of the MgF₂—CaF₂ binarysystem raw material tended to have a higher relative density by theorder of 0.5%-1.5% than those of MgF₂ simple in a range of goodsintering conditions.

When observing the sections of those sintered bodies, in the case ofsintered bodies sintered at temperatures lower than 900° C., not manybut some open pores were noticed in some of them, wherein the bridgewidth of the sintered portion was narrow, so that it could be regardedas absolutely insufficient progress of sintering.

In the case of sintered bodies sintered at temperatures of 1160° C. ormore, especially 1200° C. or more, those had a porous pumiceousstructure as if bubbles were innumerably formed inside. Fine bubbleswhich were almost perfect spheres of several to dozen μm in diameterwere observed all over the sintered body and innumerable irregularbubbles (foaming bubbles and aggregates thereof) of 10 μm or more indiameter were found all over the sections.

From another examination using a differential thermal analyzer by thepresent inventors, it was found out that when heating the compound ofMgF₂—CaF₂ binary system, the weight clearly started to decrease at atemperature of about 800° C.-850° C. (the temperature becomes graduallyhigher within the temperature range as the mix proportion of CaF₂ toMgF₂ increases), and that the weight started to drastically decrease atabout 850° C.-900° C. This means that a sublimation phenomenon in whichMgF₂ or CaF₂ dissolves/vaporizes to generate fluorine starts due toheating at about 800° C.-850° C. or more.

A foaming phenomenon through this fluorine sublimation is noticeablycaused by heating at about 850° C.-900° C. or more, and fine bubbles areformed all over the sintered body. The behavior of the foaming bubblessuch as defoaming or remaining as bubbles is decided according to thedegree of progress of the sintering step, in which portion of thesintered body they were formed and the like. In the primary flocculationprocess, for example, since the whole sintered body contains mainly openpores, the majority of foaming bubbles can be defoamed through the openpores, leading to few bubbles left. In the secondary flocculationprocess, since the sintered body contains mainly closed pores, a largenumber of foaming bubbles cannot be defoamed, leading to remaining asbubbles. To swiftly complete the sintering in the secondary flocculationprocess leads to suppressing foaming and reducing residual bubbles.

Hence, it is preferable that the transition from the primaryflocculation process to the secondary flocculation process should beadvanced in the whole sintered body with as small a difference of thedegree of progress as possible among the portions thereof. However, itis not easy to undergo the transition from the primary flocculationprocess to the secondary flocculation process in the whole sintered bodywithout a difference of the degree of progress among the portionsthereof.

Then, the present inventors considered the below-described method.

Heating at a relatively low temperature in the temperature limits justbelow the starting temperature of foaming Tn (850° C.-900° C.),specifically in the temperature limits between (Tn−100° C.) and Tn for arelatively long period of time was conducted, so that the primaryflocculation process and the first half of the secondary flocculationprocess were completed. And then, by heating at a temperature in thevicinity of the temperature (980° C.) at which a solid solution startsto be formed for a relatively short period of time, the second half ofthe secondary flocculation process was completed. By such sintering, thedegree of progress of sintering could be made uniform in the wholesintered body, and the formation of bubbles could be suppressed as muchas possible.

How the proper sintering conditions were decided is described below.

In the same manner as the above sintering condition change test, a mainraw material MgF₂ was mixed with CaF₂ of 3% by weight. CMC of 0.1% byweight was added thereto as a sintering aid. The same was molded using amold form for a large-size test at a molding pressure of 20 MPa of auniaxial press device and a molding pressure of 20 MPa of a CIP device.Preliminary sintering was conducted on this CIP molded body at 650° C.which was held for 6 hours in an air atmosphere.

As the conditions of the sintering step, the atmosphere was set to be anitrogen gas atmosphere. Preliminary tests concerning each of heatingand cooling conditions in the heating pattern were conducted in threecases of the required time of 4, 6 and 8 hours. As a result, in the caseof 4 hours, small cracks occurred in the sintered body, while in theother cases, the results were good. Therefore, the required time was setto be 6 hours, shorter one selected from 6 and 8 hours.

The atmosphere was set to be a nitrogen gas atmosphere, and the heatingtemperature was varied in a range of 700° C. to 1250° C. In eleven casesof the holding time of 2, 3, 4, 5, 6, 8, 10, 12, 14, 16 and 18 hours,the tests were conducted.

As a result, in the case of less than 750° C., the compactness wasinsufficient, regardless of the holding time. In the case of heating at750° C., the compactness was insufficient with a holding time of 4 hoursor less. On the other hand, in the case of heating at 1160° C. or more,a large number of bubbles were generated due to too fast sinteringspeed, regardless of the holding time. In the case of a holding time of18 hours, in some cases, foaming occurred in part of the periphery ofthe sintered body, leading to getting out of shape in appearance.

Reviewing the results, in the case of heating at 750° C., the sinteringstate was good with a holding time of 14 and 16 hours.

In the case of heating at 800° C., the sintering state was good with aholding time of 10 and 12 hours, while slightly insufficient with 6 and8 hours, and beyond decision of quality with 14 hours or more.

In the case of 830° C., the sintering state was good with a holding timeof 10 and 12 hours.

In the case of 850° C., the sintering state was good with a holding timeof 8, 10 and 12 hours, while slightly insufficient with 5 hours, andbeyond decision of quality with 14 hours or more.

In the case of 900° C., the sintering state was good with a holding timeof 5 to 12 hours, while slightly insufficient with 4 hours, and beyonddecision of quality with 14 hours or more.

In the case of 1000° C., the sintering state was good with a holdingtime of 5 to 12 hours, while slightly insufficient with 4 hours, andmuch foaming was seen with 14 hours or more.

In the case of 1050° C., the sintering state was good with a holdingtime of 5 to 10 hours, while slightly insufficient with 4 hours, andmuch foaming was seen with 12 hours or more.

In the case of 1100° C., the sintering state was good with a holdingtime of 4 to 8 hours, while slightly insufficient with 3 hours or less,and much foaming was seen with 10 hours or more.

In the case of 1150° C., the sintering state was good with a holdingtime of 2 and 3 hours, while much foaming was seen with 4 hours or more.

In the case of 1160° C. or more, much foaming was seen with any holdingtime, and the results were beyond decision of quality or poor because oftoo much melting.

Here, when the heating temperature was a comparatively low temperatureof 750° C. to 850° C., the sintering state was good with a holding timeof 6 to 12 hours, while that was slightly insufficient with a holdingtime of 3 to 5 hours. Since the method according to the preferredembodiment has the subsequent secondary sintering step, with theevaluation in this step (equivalent to the primary sintering step), theholding time of 3-12 hours was regarded as a good heating condition.

In order to examine the relationship between the heating temperature andthe bulk density of the sintered body, using the same preliminarysintered bodies as the above, the heating temperature was varied withina range of 600° C. to 1300° C. (with a holding time of 6 hours in anycase).

As a result, in the case of a heating temperature of 850° C., the bulkdensity was approximately 2.96 g/cm³ (the relative density of 94.0%).The sintered body having a bulk density of that value or more was judgedto have sufficient compactness without troubles such as losing its shapein the treatment of the second step. On the other hand, in the case ofheating temperatures of 1160° C. or more, in some cases, foamingoccurred in part of the periphery of the sintered body, resulting in atrouble such as getting out of shape in appearance.

From the above examination results of the sintering conditions and therelationship between the heating temperature and the bulk density, itwas judged that, if the sintering step was one heating step, the heatingtemperature of 850° C. to 1150° C. and the holding time of 3 to 12 hourswere proper.

What was clarified here is, when relatively long time heating, such asat 900° C. for 14 hours or more, at 1000° C. for 14 hours or more, at1100° C. for 10 hours or more, or at 1150° C. for 8 hours or more, wasconducted, a large number of foaming bubbles were generated and part ofthose gathered and grew to large bubbles. Such sintered body involveddefects which would cause cracks to occur from a large bubble portion orcause splitting in processing of the next mechanical processing step.

From these situations, as a fundamental plan of the sintering step, itwas decided that foaming should be suppressed as much as possible, whilethe sintering reaction should be allowed to sufficiently make progress,leading to producing a sintered body having a good processability in thesubsequent mechanical processing step.

At the first stage of the sintering step (the primary sintering step),it was aimed to suppress foaming to a minimum, to allow the sintering tomake slow progress, and to minimize a difference of the degree ofprogress of sintering between the inside portion and the peripheryportion of the sintered body.

Therefore, the heating temperature was decided to be within the aboverange of 700° C. to 1150° C. Since the starting temperature of foamingTn is 850° C. in the case of a raw material mainly comprising MgF₂, itwas judged that the heating temperature should be 850° C. or less, notexceeding the temperature. On the other hand, since the sintering statewas insufficient in the case of heating at temperatures lower than theTn by 100° C. or more, it was judged that the heating temperature at thefirst stage of the sintering step should be between (Tn−100° C.) and Tn,between 750° C. and 850° C. in the case of a raw material mainlycomprising MgF₂.

The proper heating conditions in the primary sintering step were theheating temperature between (Tn−100° C.) and Tn, and the holding time of3-12 hours. The same tendency was found even in cases where the mixproportion of CaF₂ to MgF₂ varied between 0-90% by weight.

Heating at the stage of enhancing the sintering reaction of the sinteredbody, that is, heating in the secondary sintering step, was decided tobe conducted properly in the temperature limits in the vicinity of 980°C. at which a solid solution starts to be formed, that is, 900° C. to1150° C. It was aimed to make the holding time as short as possible inorder to enhance the sintering reaction and suppress foaming. The properholding time was decided to be 0.5 to 8 hours, since the enhancement ofthe sintering reaction was poor in the case of less than 0.5 hour, andtoo many bubbles were formed in the case of 9 hours or more.

The examination of the proper conditions of the heating temperature andthe holding time in the secondary sintering process when the atmosphericgas was changed from nitrogen gas to helium gas is described below.

A mixture of a main raw material MgF₂ with CaF₂ of 3% by weight was usedas a starting material, to which CMC of 0.1% by weight was added as asintering aid.

Using a mold form of press molding for a large-size test, the materialwas molded at a molding pressure of 20 MPa of a uniaxial press deviceand at a molding pressure of 20 MPa of a CIP device. This CIP moldedbody was preliminary sintered at 650° C. for 6 hours in an airatmosphere so as to obtain a preliminary sintered body.

Using helium gas as the atmospheric gas in the primary and secondarysintering processes, the preliminary sintered body was heated to 840° C.which was held for 6 hours as primary sintering. Then, it was raised toeach of secondary sintering temperatures varying in a range of 700° C.to 1250° C., at an interval of every 50° C. for 2 hours, and the targettemperature was held for 2 hours. And then, the heating was stopped andthe temperature was lowered by self-cooling (so-called furnace cooling)for about 20 hours, and when reached a predetermined taking-outtemperature of 100° C. or lower, the sintered body was taken out.

As a result of the sintering test with the above two-stage sinteringstep, in the case of a temperature range of 900° C. to 1150° C., most ofthe sintered bodies had a high bulk density exceeding 2.96 g/cm³. Thetrue density of this binary system compound (mixed with CaF₂ of 3% byweight) was 3.15 g/cm³ and the relative density thereof was 94.0%. Thetrue density and the relative density thereof were the same as the truedensity and the relative density of the raw material of MgF₂ simple.

In either case of sintering temperatures lower than 900° C., and thoseof 1160° C. or more, the relative density was lower than 94.0% (the bulkdensity of 2.96 g/cm³). The sintered bodies sintered in a helium gasatmosphere tended to have a higher relative density within a range ofgood sintering conditions (a temperature range of 900° C. to 1150° C.)by the order of 0.5%-1% than in a nitrogen gas atmosphere.

It is considered that the reason why the bulk density becomes high in ahelium gas atmosphere is because the diffusion velocity of helium gaswithin the bulk (parent) of the sintered body is higher than that ofnitrogen gas. It is presumed that, since helium gas more easily diffuseswithin the bulk than nitrogen gas, when voids become closed pores withthe progress of sintering in the sintering process, part of the closedpores disappear without becoming bubbles, or the sizes of the closedpores become smaller.

However, helium gas showed better effects within a range of the aboveproper sintering conditions, while the effects were not all-around,being not noticeably seen in the region other than the proper sinteringconditions.

As the reasons of such result, it was considered that under thesintering conditions outside the proper range, for example, there was alimit in improving too slow sintering speed due to an insufficientheating condition, or in the case of an excessive heating condition, theununiformity of the sintering speed of every part of the sintered bodycould not be improved by enhancing the diffusivity of helium gas in thebulk.

In the case of helium gas, when the heating temperature in the sinteringstep was less than 900° C., regardless of the holding time, or in thecase of a holding time of 4 hours or less, the compactness wasinsufficient. When the heating temperature was 1160° C. or more, thesintering speed was too high, regardless of the holding time, as is thecase with nitrogen gas, resulting in occurrence of a large number ofbubbles, and in the case of a holding time of 16 hours or more, becauseof foaming, the appearance got out of shape in some cases.

Accordingly, in the case of a starting raw material made by mixingmainly MgF₂ with CaF₂, it was judged that the proper range of sinteringtemperatures was 900° C.-1150° C., regardless of the kind of inertatmospheric gas in the sintering step. Furthermore, in the case ofsintering temperatures of 930° C.-1100° C., even when the sintered bodywas provided to the mechanical processing, structural defects such ascracks were difficult to occur, leading to good mechanicalprocessability. As a result, it was judged that the sinteringtemperature was more preferably in a temperature range of 930° C.-1100°C.

Therefore, as proper heating conditions of the sintering step in ahelium gas atmosphere, as is the case with the above nitrogen gasatmosphere, the proper condition of the primary sintering step was in arange of 750° C. or more and less than the starting temperature offoaming, while that of the secondary sintering step was in a temperaturerange of 900° C.-1150° C.

The inert gas is not limited to nitrogen and helium. In the case ofargon or neon, the same effects can be obtained. Moreover, since neon isexpected to have high solubility or high diffusivity in the parent ofthe sintered body, like helium, the defoaming phenomenon can be morepromoted and effects equal to those of helium can be expected.

In this invention, on the basis of the above technical ideas, the rawmaterial particles were further pulverized, and using a uniaxialpressing hot press furnace or a hot isostatic pressing (HIP) furnace inaddition to an atmospheric firing furnace as furnaces used in thesintering step, defoaming of the sintered body was further promoted, andas a result, a higher-density sintered body could be successfullyobtained.

Particularly what was found is:

(i) when the particle size distribution of the raw material is narrowand the particle size distribution curve thereof is approximate to anormal distribution, the smaller the mean particle diameter thereof is,the higher packing density the molded body thereof (for example,compared with that of the raw material having a median diameter of about8 μm or more, that of the raw material having a median diameter of about6 μm or less noticeably) tends to have, and the easier it becomes tocompact the preliminary sintered body and the sintered body;

(ii) in sintering in a light element gas atmosphere such as He or Neamong inert gases, the lighter the element is, the more easily itdiffuses in pores or in the bulk of the sintered body, so that acapillary phenomenon and a defoaming phenomenon are promoted. Bubblesare hard to remain and compacting becomes easy to conduct; and

(iii) in the sintering step using a uniaxial hot press furnace and a HIPfurnace, particularly in the secondary sintering step, by applyingpressure to the sintered body in heating, defoaming of the sintered bodycan be promoted.

By proving these findings and further promoting defoaming of sinteredbodies, higher-density sintered bodies could be successfully obtained.

The Description of the Particle Size Distribution Performance Test

In the case of a raw material of MgF₂ simple, selecting the rawmaterials having the following three kinds of particle sizedistributions, respectively, the test was conducted. That is, as shownin FIG. 4, as the three kinds of particle size distributions, particlesize distribution A (about 8 μm in median diameter), particle sizedistribution B (similarly about 6 μm) and particle size distribution C(similarly about 4 μm), were adopted.

As a sintering aid, CMC of 0.1% by weight was added thereto and mixed toobtain a starting raw material.

This starting raw material was filled into the mold form for alarge-size test of a press device, and press molded at a uniaxial presspressure of 20 MPa.

This press molded body (the size about 220 mm×220 mm×t85 mm) was putinto a thick vinyl bag, which was then deaired and sealed, and it wasput into the molding part of a cold isostatic pressing (CIP) device(inside size: 350 mm in diameter×120 mm in height). The space betweenthe vinyl bag with the press molded body therein and the molding part ofthe CIP device was filled with clean water, and then, isostatic pressingwas conducted at a hydraulic pressure of 20 MPa at room temperature soas to form a CIP molded body.

This CIP molded body was heated (preliminary sintered) at 650° C. for 6hours in an air atmosphere so as to obtain a preliminary sintered body.

As primary sintering, using an atmospheric firing furnace in which theatmosphere can be adjusted, in a He gas atmosphere, the preliminarysintered body was heated from room temperature to 840° C. at a fixedrate for 6 hours, and the temperature was held there for 6 hours (theprimary sintering condition was fixed).

Then, the temperature was raised to a secondary sintering temperature(varied in a range of 700° C. to 1250° C., at an interval of every 50°C.) at a fixed rate for 2 hours, and the temperature was held there for2 hours.

The heating was then stopped and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours, and afterconfirming that the temperature reached a predetermined taking-outtemperature of 100° C. or lower, the sintered body was taken out.

The sintering test by such two-stage sintering step was conducted. Here,since it has been proved that the proper range of the heating andholding time in this secondary sintering step is 0.5-3 hours asdescribed above, the holding time can be changed within this range. Theresults in the case of a holding time of 2 hours are shown in FIG. 4.

The bulk densities of the sintered bodies, the starting raw materials ofwhich had the particle size distributions B and C, were high to theextent that the relative densities exceeded 96% in the case of sinteringtemperatures in a range of 900° C. to 1150° C. In both cases ofsintering temperatures less than 900° C. and those of 1160° C. or more,the relative densities were lower than 96%.

As shown in FIG. 4, the difference caused by the different particle sizeconditions of the starting raw materials was made clear. In the range ofgood sintering conditions, the sintered bodies having the particle sizedistribution B (about 6 μm in median diameter) tended to have relativedensities higher by the order of 1% than the sintered bodies having theparticle size distribution A (about 8 μm in median diameter). Thesintered bodies having the particle size distribution C (about 4 μm inmedian diameter) tended to have relative densities higher by the orderof 2% than the sintered bodies having the particle size distribution A(about 8 μm in median diameter). The highest attained value of relativedensity of the sintered bodies having the particle size distribution Cexceeded 97%, which was high.

When observing the sections of those sintered bodies, in the case ofsintered bodies sintered at temperatures lower than 900° C., not manybut some open pores were noticed in some of them, wherein the bridgewidth of the sintered portion was narrow, so that it could be regardedas absolutely insufficient progress of sintering.

On the other hand, in the case of sintered bodies sintered attemperatures of 1160° C. or more, especially 1200° C. or more, those hada porous pumiceous structure as if bubbles were innumerably formedinside. Fine bubbles which were almost perfect spheres of several todozen μm in diameter were observed all over the sintered body andinnumerable irregular bubbles of 10 μm or more in diameter were foundall over the sections. That is, a large number of bubbles which appearedto be foaming bubbles and aggregates thereof were noticed.

In the case of a raw material of MgF₂—CaF₂ binary system, as is the casewith a raw material of MgF₂ simple, selecting the raw materials havingthree kinds of particle size distributions, respectively, the test wasconducted. That is, as shown in FIG. 5, as the three kinds of particlesize distributions, particle size distribution A (about 8 μm in mediandiameter), particle size distribution B (similarly about 6 μm) andparticle size distribution C (similarly about 4 μm), were adopted.

As a sintering aid, CMC of 0.1% by weight was added thereto and mixed toobtain a starting raw material.

This starting raw material was filled into the mold form for asmall-size test of a press device, and press molded at a uniaxial presspressure of 20 MPa.

This press molded body was put into a thick vinyl bag, which was thendeaired and sealed, and it was put into the molding part of a coldisostatic pressing (CIP) device. The space between the vinyl bag withthe press molded body therein and the molding part of the CIP device wasfilled with clean water, and then, isostatic pressing was conducted at amolding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

This CIP molded body was heated (preliminary sintered) at 650° C. for 6hours in an air atmosphere so as to obtain a preliminary sintered body.

As primary sintering, using an atmospheric firing furnace in which theatmosphere can be adjusted, in a He gas atmosphere, the preliminarysintered body was heated from room temperature to 840° C. at a fixedrate for 6 hours, and the temperature was held there for 6 hours (theprimary sintering condition was fixed).

Then, the temperature was raised to a secondary sintering temperature(varied in a range of 700° C. to 1250° C., at an interval of every 50°C.) at a fixed rate for 2 hours, and the temperature was held there for2 hours.

The heating was then stopped and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours, and afterconfirming that the temperature reached a predetermined taking-outtemperature of 100° C. or lower, the sintered body was taken out.

The sintering test by such two-stage sintering step was conducted. Here,since it has been proved that the proper range of the heating andholding time in this secondary sintering step is 0.5-8 hours asdescribed above, the holding time can be changed within this range. Theresults in the case of a holding time of 2 hours are shown in FIG. 5.

The bulk densities of the sintered bodies, the starting raw materials ofwhich had the particle size distributions B and C, were high to theextent that the relative densities exceeded 96% in the case of sinteringtemperatures in a range of 900° C. to 1150° C. In both cases ofsintering temperatures less than 900° C. and those of 1160° C. or more,the relative densities were lower than 96%.

As shown in FIG. 5, the difference caused by the different particle sizeconditions of the starting raw materials was made clear. In the range ofgood sintering conditions, the sintered bodies having the particle sizedistribution B (about 6 μm in median diameter) tended to have relativedensities higher by the order of 1% than the sintered bodies having theparticle size distribution A (about 8 μm in median diameter). Thesintered bodies having the particle size distribution C (about 4 μm inmedian diameter) tended to have relative densities higher by the orderof 2% than the sintered bodies having the particle size distribution A(about 8 μm in median diameter). The highest attained value of relativedensity of the sintered bodies having the particle size distribution Cexceeded 97%, which was high.

When observing the sections of those sintered bodies, in the case ofsintered bodies sintered at temperatures lower than 900° C., not manybut some open pores were noticed in some of them, wherein the bridgewidth of the sintered portion was narrow, so that it could be regardedas absolutely insufficient progress of sintering.

On the other hand, in the case of sintered bodies sintered attemperatures of 1160° C. or more, especially 1200° C. or more, those hada porous pumiceous structure as if bubbles were innumerably formedinside. Fine bubbles which were almost perfect spheres of several todozen μm in diameter were observed all over the sintered body andinnumerable irregular bubbles of 10 μm or more in diameter were foundall over the sections. That is, a large number of bubbles which appearedto be foaming bubbles and aggregates thereof were noticed.

Thus, it was proved that by controlling the particle size distributioncondition of the starting material, further compacting of the sinteredbody could be achieved.

The Description of the Pressure Firing Test

As firing furnaces used in these primary and secondary sintering steps,three kinds of firing furnaces, an atmospheric firing furnace in whichthe atmosphere can be adjusted, a uniaxial pressing hot press furnaceand a hot isostatic pressing (HIP) furnace, were prepared. By usingthem, with the same heating temperature conditions, heating wasconducted.

Specifically, heating was conducted:

(a) using the atmospheric firing furnace, in an air or in an atmosphericinert gas atmosphere;

(b) using the uniaxial pressing hot press furnace, in an inert gasatmosphere or in a vacuum atmosphere of less than 100 Pa; and

(c) using the HIP furnace, in an inert gas atmosphere.

The reason why the degree of vacuum of the uniaxial pressing hot pressfurnace was set to be less than 100 Pa is because with low degrees ofvacuum of 100 Pa or more, the compactness of the sintered bodies tendedto be insufficient, compared with in an inert gas atmosphere or in ahigh vacuum of less than 100 Pa.

It was considered that the presence or absence of pressure applicationor pressure reduction to the sintered bodies during firing, and thedifference of the mode of pressure application caused differences indensity of the sintered bodies.

That is, in the case of the uniaxial pressing hot press furnace, byuniaxially driving a press jig, pressure can be applied, differentlyfrom the atmospheric firing furnace. In the driven press jig sideportion of the sintered body, defoaming can be promoted.

In the case of the HIP furnace, by increasing the gas pressure thereinto a prescribed pressure, the sintered body can be pressed from triaxialdirections. Therefore, it was considered that defoaming of the sinteredbody could be further promoted compared with the atmospheric firingfurnace and the uniaxial pressing hot press furnace.

As starting materials, raw materials having the particle sizedistribution B (about 6 μm in median diameter) shown in FIG. 4 (MgF₂simple raw material) and in FIG. 5 (MgF₂—CaF₂ binary system rawmaterial) were used. Preliminary sintered bodies formed with the sameconditions in uniaxial press molding, CIP molding and preliminarysintering were used. In the subsequent firing, in place of anatmospheric firing furnace, primary sintering and secondary sinteringusing an uniaxial pressing hot press furnace or a HIP furnace wereconducted.

The heating conditions in the primary sintering were fixed to a heatingtemperature of 840° C. in a nitrogen gas atmosphere and a holding timeof 6 hours, while the heating temperature in the secondary sinteringwere varied (the heating and holding time was fixed to 2 hours) so as tosee changes in relative density of the sintered bodies.

Concerning pressure application in the uniaxial pressing hot pressfurnace and the HIP furnace, in the uniaxial pressing hot press furnace,a press pressure of 50 MPa was uniaxially applied during the heating andholding time, while in the HIP furnace, the gas pressure therein was setto be 200 MPa during the heating and holding time, by which a pressureof 200 MPa was substantially applied in the triaxial directions so as toconduct pressure sintering. The results showing the relationship betweenthe secondary sintering temperatures and the relative densities areshown in FIG. 6 (MgF₂ simple raw material) and FIG. 7 (MgF₂—CaF₂ binarysystem raw material).

As is obvious from FIGS. 6 and 7, which kind of firing furnace was usedgreatly influenced the relative densities of the sintered bodies. It wasproved that regardless of which raw material was used, by using theatmospheric furnace, the uniaxial pressing hot press furnace, and theHIP furnace in ascending order, the relative densities of the sinteredbodies increased. By using the uniaxial pressing hot press furnace, theheating temperature limits (proper heating temperature limits) could bewidened, and those could be further widened by using the HIP furnace.

As described above, it was found out that with the proper conditions inthe secondary sintering, that is, a heating temperature of 900° C.-1150°C., and the pressure condition of 50 MPa or more during the heating andholding time in the uniaxial pressing hot press furnace, or 200 MPa ormore, preferably 300 MPa or more during the heating and holding time inthe HIP furnace, high-density sintered bodies having a relative densityof 95% or more could be obtained. Consequently, here, these heatingcondition and pressure condition are called ‘the proper conditions’ inthe pressure sintering method.

The proper conditions with which on the sintered body obtained throughthe primary and secondary sintering steps (hereinafter, referred to as‘a secondary sintered body’), tertiary sintering using the uniaxialpressing hot press furnace or the HIP furnace is further conducted aredescribed below.

Depending on whether the relative densities of the ‘secondary sinteredbodies’ are (i) 95% or more, or (ii) less than 95%, the densities of thesintered bodies obtained after the tertiary sintering are greatlydifferent.

In the case of (i) 95% or more, when the tertiary sintering wasconducted with the proper conditions, the relative densities of thesintered bodies could be 97.5%-100%, being roughly approximate to thetheoretical density (true density) and high.

On the other hand, in the case of (ii) less than 95%, even if thetertiary sintering was conducted with the proper conditions, therelative densities of the sintered bodies were far lower than thetheoretical density (true density). Some sintered bodies had lowrelative densities not reaching 95%.

In the case of (i) 95% or more, the number of bubbles was small becauseof such high density, the number of open pores on the surface of thesintered body was also small, and the number of closed pores within thesintered body was also small. It was considered that by the pressuresintering in the tertiary sintering, most of those bubbles disappeared.

On the other hand, in the case of (ii) less than 95%, the number ofbubbles was large because of such low density, and there were manylarge-size bubbles. It was considered that even if the pressuresintering in the tertiary sintering was conducted under the properconditions, many of these bubbles remained, resulting in insufficientdensification.

EXAMPLES

The MgF₂ system fluoride sintered body for a radiation moderator and theproducing method thereof according to the present invention aredescribed below in more detail by reference to Examples. The scope ofthe present invention is not limited to the below-described Examples.

Here, a characteristic evaluation test conducted on sintered bodies isdescribed. Samples for evaluation were prepared by prototypinglarge-size sintered bodies (rough size of the sintered body: about 205mm×about 205 mm×H about 70 mm) and conducting mechanical processing suchas cut-out in the shape of a required sample thereon.

In order to evaluate the neutron moderation performance, as shown in theabove Non-Patent Documents 1 and 2, a beam emitted from an acceleratorwas allowed to collide with a plate made of Be being a target, and bynuclear reaction, high-energy neutrons (fast neutrons) were mainlygenerated.

Then, using Pb and Fe each having a large inelastic scattering crosssection as a moderator in the first half of moderation, the fastneutrons were moderated to some extent (approximately, up to 1 MeV)while suppressing the attenuation of the number of neutrons.

The moderated neutrons were irradiated to a moderator to be evaluated (amoderator in the second half of moderation), and by examining theneutrons after moderation, the moderator was evaluated.

The examination of the neutrons was conducted according to the methoddescribed in the above Non-Patent Document 3′.

The moderators to be evaluated were made of raw materials MgF₂ and CaF₂in some varied mix proportions. Through the above-described raw materialmixing step, molding step and sintering step, a high-density MgF₂—CaF₂binary system sintered body having a relative density in a fixed range(96.0±0.5%) was produced. The total thickness of a moderator in thesecond half was set to be 600 mm in any case.

What was evaluated here is the dose of epithermal neutrons havingintermediate-level energy which is effective for therapy, and how manyfast neutrons and gamma-rays having high-level energy which has a highpossibility of adversely influencing a patient (side effects), remainedin the neutrons moderated by the moderator. The results are shown inTable 1 of FIG. 9.

The dose of epithermal neutrons effective for therapy tended to slightlyincrease, as CaF₂ was added more to MgF₂, but the digit of the neutronflux (dose) of epithermal neutrons was the ninth power in any case, sothat regardless of the mix proportion of CaF₂, the dose thereofsufficient for therapy was secured.

On the other hand, the mix rate of fast neutrons having a highpossibility of adversely influencing a patient (the ratio of fastneutron dose in the total neutron dose after passing through amoderator) was the lowest in the case of mixing CaF₂ of 0 to 10% byweight (0 wt. %, 2 wt. %, 5 wt. %, 8 wt. %. and 10 wt. % as the data).It gradually increased as the mix proportion thereof far exceeded thesemix proportions and increased to 20% by weight, and to 40% by weight. Itwas the highest when CaF₂ was 100% by weight.

The mix rate of gamma-rays having the next highest possibility ofadversely influencing a patient after fast neutrons (the ratio ofgamma-ray dose in the total neutron dose after passing through amoderator) was a low digit of E⁻¹⁴ (the minus 14th power), regardless ofthe mix proportion of CaF₂ to MgF₂. It can be said that the mix rate ofgamma-rays was not greatly affected by the mix proportion of CaF₂.

From these results, it was proved that when MgF₂ was mixed with CaF₂ of0-10% by weight, it had the most excellent performance as a moderator.Even if the mix proportion was other than such mix proportions, forexample, 10.1% by weight or more and 90% by weight or less, the neutronson the level usable for therapy were included, resulting in holdingrequired characteristics in moderation performance.

The evaluation results are limited to the cases where the relativedensity of the sintered body is roughly within a fixed range(96.0±0.5%). The higher relative density the sintered body has, thelower the residual dose and the mix rate of fast neutrons are.Conversely, the lower relative density the sintered body has, the higherthe residual dose and the mix rate of fast neutrons are. Accordingly,the improvement of the density of the sintered body is effective for amoderator.

As described above, concerning the moderation performance of a moderatorto neutrons, it was sufficient that sintered bodies of MgF₂ simple andMgF₂—CaF₂ binary system having a compact structure should have just arelative density of 96% or more.

The moderator to neutrons is required to have mechanical strength otherthan moderation performance. According to the below-describedexamination of mechanical strength, the MgF₂ system fluoride sinteredbody for a radiation moderator according to the Examples had sufficientmechanical strength, with which it could be used without problems inprocessing and molding such as cutting-off, grinding, polishing,cleaning and drying as a moderating member in a moderation system devicefor BNCT, and further in handling such as the installation thereof inthe moderation system device. Even if it was irradiated with neutrons,it was capable of resisting their irradiation impacts, being extremelyexcellent.

As mechanical strengths, bending strength and Vickers hardness wereexamined. The samples for bending strength, having a size of 4 mm×46mm×t3 mm with the upper and lower surfaces optically polished wereprepared according to JIS C2141, and tested according to the three-pointbending test JIS R1601.

To obtain the Vickers hardness, according to JIS Z2251-1992, using‘Micro Hardness Tester’ made by Shimadzu Corporation, an indenter havinga load of 100 g was pressed for 5 seconds of loading time so as tomeasure the diagonal length of the impression, which was converted intohardness in the following manner.Vickers hardness=0.18909×P/(d)²

Here, P: load (N) and d: diagonal length of impression (mm)

The pressure applied in the pressure sintering step was 50 MPa inuniaxial pressing hot press, or 200 MPa in HIP in every below-describedExample.

The pressure applied in the pressure sintering step was 25 MPa inuniaxial pressing hot press, or 100 MPa in HIP in every ComparativeExample.

Example 1

A high-purity MgF₂ raw material powder (median diameter: about 140 μmand purity of 99.9% by weight or more) was pulverized using the abovealumina pot mill and alumina balls for two weeks (median diameter of thepulverized simple raw material: about 5 μm and purity of 99.9% by weightor more).

Thereafter, a carboxymethyl cellulose (CMC) solution was added theretoas a sintering aid in the proportion of 0.5% by weight to 100 of thecompound, which was mixed using a V mixer for 12 hours so as to obtain astarting raw material.

This starting raw material was filled into a mold form (inside size ofthe mold form of 220 mm×220 mm×H150 mm) of a uniaxial press device, andcompressed and molded at a uniaxial press pressure of 20 MPa.

This press molded body (size of about 220 mm×220 mm×t85 mm), which wasput into a thick vinyl bag and sealed after deairing, was put in themolding part (inside size: dia. 350 mm×H120 mm) of a cold isostaticpressing (CIP) device. Clean water was filled into the space between thevinyl bag with the press molded body therein and the CIP molding part,and by isostatic pressing at a molding pressure of 20 MPa at roomtemperature, CIP molding was conducted.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a helium gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 1050° C. at a fixed rate for 4 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to 100° C.or lower at which time it was previously set to take out the sinteredbody. After confirming that, it was taken out.

The bulk density of the sintered body, that is, ‘the secondary sinteredbody’ was calculated at 3.075 g/cm³ (the true density of this compoundis 3.15 g/cm³ and the relative density thereof is 97.6%. Hereinafter,referred to as “true density of 3.15 g/cm³ and relative density of97.6%”) from the bulk volume of the appearance thereof and the weightthereof. The sintering state thereof was good.

Since the appearance of the sintered body was a square form, the “bulkdensity” here was obtained by a method wherein the bulk volume wascalculated from the measured two sides of the square and thickness, andthe weight separately measured was divided by the bulk volume. This alsoapplied to the following.

Using a sample taken from this sintered body, evaluation tests ofneutron moderation performance and characteristics of every kind wereconducted. The results are shown in Table 2 of FIG. 10.

As for the following Examples and Comparative Examples, the evaluationtests of neutron moderation performance and characteristics of everykind were conducted in the same manner and the results are shown inTable 2 of FIG. 10. Here, concerning a sintered body of CaF₂ simple,being a comparative material, the neutron moderation performance andmechanical strengths were measured in the same manner.

The sintered body according to Example 1 showed excellent neutronmoderation performance, and the mechanical strengths thereof were alsogood enough not to cause problems in handling in the next step.

Example 2

Using a MgF₂ simple powder (median diameter of about 5 μm and purity of99.9% by weight or more) obtained by pulverizing a high-purity MgF₂ rawmaterial powder with the same pulverization conditions as in the aboveExample 1, a sintering aid was added thereto so as to obtain a startingraw material like the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. By conducting isostatic pressing of 20 MPa on this pressmolded body (the size of about 220 mm×220 mm×t85 mm) using a CIP devicein the same manner as in the above Example 1, a CIP molded body wasformed.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

As HIP treatment, this preliminary sintered body was heated from roomtemperature to 840° C. at a fixed rate for 6 hours in a nitrogen gasatmosphere using a HIP furnace, while the gas pressure in the furnacewas set at 200 MPa, and the temperature was held there for 8 hours. Itwas then raised to 1020° C. at a fixed rate for 4 hours while the gaspressure therein was held at 200 MPa, and the temperature was held therefor 2 hours.

The heating was then stopped, while the gas pressure therein wasgradually decreased, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body, that is, the secondary sintered body was taken out.

The relative density of the secondary sintered body was 98.4% (bulkdensity of 3.11 g/cm³ and true density of 3.16 g/cm³), and the sinteringstate thereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 3

A high-purity MgF₂ raw material powder (median diameter: about 140 μmand purity of 99.9% by weight or more) was mixed with a high-purity CaF₂raw material powder (median diameter: about 140 μm and purity of 99.9%by weight or more) of 1.5% by weight, and the compound was pulverizedusing the above alumina pot mill and alumina balls for two weeks (mediandiameter of the pulverized compound: about 5 μm and purity of 99.9% byweight or more).

Thereafter, a sintering aid was added to the pulverized compound so asto obtain a starting raw material like the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. By conducting isostatic pressing of 20 MPa on this pressmolded body (the size of about 220 mm×220 mm×t85 mm) using a CIP devicein the same manner as in the above Example 1, a CIP molded body wasformed.

Heating was conducted on this CIP molded body at 650° C. for 6 hours inan air atmosphere so as to obtain a preliminary sintered body.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 8hours. It was then raised to 1050° C. at a fixed rate for 2 hours, andthe temperature was held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body (secondary sintered body) was96.5% (bulk density of 3.04 g/cm³ and true density of 3.15 g/cm³), andthe sintering state thereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 4

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of1.5% by weight and pulverized for two weeks in the same manner as in theabove Example 1. The pulverized compound (median diameter of thepulverized compound: about 5 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a press device in the same manner as in the above Example 1.This press molded body was put in the molding part of a cold isostaticpressing (CIP) device, and by conducting CIP molding thereon at amolding pressure of 20 MPa at room temperature, a CIP molded body wasformed.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 820°C. at a fixed rate for 6 hours in a vacuum the degree of which is 50 Pausing a uniaxial hot press furnace, and the temperature was held therefor 6 hours. It was then raised to 930° C. at a fixed rate for 2 hours,and the temperature was held there for 4 hours. In each of these holdingtimes, pressing with a load of 50 MPa was uniaxially conducted.

Then, the pressing and heating were stopped, and the temperature waslowered by self-cooling (so-called furnace cooling) for about 10 hoursto a predetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 97.5% (bulk density of3.07 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 5

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of1.5% by weight and pulverized for two weeks in the same manner as in theabove Example 1. The pulverized compound (median diameter of thepulverized compound: about 5 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody. As HIP treatment, this preliminary sintered body was heated fromroom temperature to 840° C. at a fixed rate for 6 hours in a nitrogengas atmosphere using a HIP furnace, while the gas pressure therein wasset at 200 MPa, and the temperature was held there for 5 hours. It wasthen raised to 1150° C. at a fixed rate for 2 hours while keeping thegas pressure, and the temperature was held there for 0.5 hour.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body was taken out.

The relative density of the sintered body was 97.8% (bulk density of3.08 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 6

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of1.5% by weight and pulverized for three weeks in the same manner as inthe above Example 1. The pulverized compound (median diameter of thepulverized compound: about 4 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody. This preliminary sintered body was heated from room temperature to840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere usingan atmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 1020° C. at a fixed rate for 2 hours, andthe temperature was held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 97.1% (bulk density of3.06 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 7

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of1.5% by weight and pulverized for one week in the same manner as in theabove Example 1. The pulverized compound (median diameter of thepulverized compound: about 6 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody. This preliminary sintered body was heated from room temperature to840° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere usingan atmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 1050° C. at a fixed rate for 2 hours, andthe temperature was held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 96.2% (bulk density of3.03 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 8

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of10% by weight and pulverized for one week in the same manner as in theabove Example 1. The pulverized compound (median diameter of thepulverized compound: about 6 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody. This preliminary sintered body was heated from room temperature to800° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere usingan atmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 960° C. at a fixed rate for 2 hours, andthe temperature was held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 95.2% (bulk density of3.00 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 9

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of20% by weight and pulverized in the same manner as in the aboveExample 1. The pulverized compound (median diameter of the pulverizedcompound: about 5 μm and purity of 99.9% by weight or more) was mixedwith a sintering aid so as to obtain a starting raw material like theabove Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody. This preliminary sintered body was heated from room temperature to840° C. at a fixed rate for 6 hours in an air atmosphere using anatmospheric firing furnace, and the temperature was held there for 8hours. It was then raised to 1150° C. at a fixed rate for 2 hours, andthe temperature was held there for 0.75 hour.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 96.8% (bulk density of3.05 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 10

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of20% by weight and pulverized for one week in the same manner as in theabove Example 1. The pulverized compound (median diameter of thepulverized compound: about 6 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody. This preliminary sintered body was heated from room temperature to810° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere usingan atmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 960° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 95.4% (bulk density of3.005 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 11

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of50% by weight and pulverized for two weeks in the same manner as in theabove Example 1. The pulverized compound (median diameter of thepulverized compound: about 5 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody. This preliminary sintered body was heated from room temperature to830° C. at a fixed rate for 6 hours in a nitrogen gas atmosphere usingan atmospheric firing furnace, and the temperature was held there for 6hours. It was then raised to 1080° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 96.2% (bulk density of3.04 g/cm³ and true density of 3.16 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 12

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of50% by weight and pulverized for two weeks in the same manner as in theabove Example 1. The pulverized compound (median diameter of thepulverized compound: about 5 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody. This preliminary sintered body was heated from room temperature to860° C. at a fixed rate for 6 hours in a helium gas atmosphere using auniaxial hot press furnace, and the temperature was held there for 8hours. It was then raised to 1080° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours. In each of these holdingtimes, pressing with a load of 50 MPa was uniaxially conducted.

Then, the pressing and heating were stopped, and the temperature waslowered by self-cooling (so-called furnace cooling) for about 10 hoursto a predetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 97.2% (bulk density of3.07 g/cm³ and true density of 3.16 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 13

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of50% by weight and pulverized for two weeks in the same manner as in theabove Example 1. The pulverized compound (median diameter of thepulverized compound: about 5 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

As HIP treatment, this preliminary sintered body was heated from roomtemperature to 860° C. at a fixed rate for 6 hours in a nitrogen gasatmosphere using a HIP furnace, while the gas pressure was set at 200MPa, and the temperature was held there for 8 hours. Thereafter, whilekeeping the gas pressure, it was raised to 970° C. at a fixed rate for 2hours, and the temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body was taken out.

The relative density of the sintered body was 97.5% (bulk density of3.08 g/cm³ and true density of 3.16 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 14

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of90% by weight and pulverized for three weeks in the same manner as inthe above Example 1. The pulverized compound (median diameter of thepulverized compound: about 4 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 830°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using auniaxial hot press furnace, and the temperature was held there for 9hours. It was then raised to 1080° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours. In each of these holdingtimes, pressing with a load of 50 MPa was uniaxially conducted.

Then, the pressing and heating were stopped, and the temperature waslowered by self-cooling (so-called furnace cooling) for about 10 hoursto a predetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 97.2% (bulk density of3.08 g/cm³ and true density of 3.17 g/cm³), and the sintering statethereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 15

A MgF₂ powder being a main raw material was pulverized with the samepulverization conditions as in the above Example 11, and the pulverizedMgF₂ simple powder (median diameter of the pulverized raw material:about 4 μm and purity of 99.9% by weight or more) was mixed with asintering aid so as to obtain a starting raw material like the aboveExample 1.

On this starting raw material, uniaxial press molding was conducted withthe same molding conditions as in the above Example 1, and CIP moldingwas further conducted thereon so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 1000° C. at a fixed rate for 4 hours, andthe temperature was held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 96.5% (bulkdensity of 3.04 g/cm³ and true density of 3.15 g/cm³), and the sinteringstate thereof was good.

This ‘secondary sintered body’ was heated from room temperature to 940°C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 200 MPa, andthe temperature was held there for 1.5 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out. The relativedensity of the ‘tertiary sintered body’ was 99.4% (bulk density of 3.13g/cm³ and true density of 3.15 g/cm³), and the sintering state thereofwas good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 16

A MgF₂ powder being a main raw material was pulverized with the samepulverization conditions as in the above Example 11, and the pulverizedMgF₂ simple powder (median diameter of the pulverized raw material:about 4 μm and purity of 99.9% by weight or more) was mixed with asintering aid so as to obtain a starting raw material like the aboveExample 1.

On this starting raw material, uniaxial press molding was conducted withthe same molding conditions as in the above Example 1, and CIP moldingwas further conducted thereon so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 1050° C. at a fixed rate for 4 hours, andthe temperature was held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the ‘secondary sintered body’ was 97.3%(bulk density of 3.065 g/cm³ and true density of 3.15 g/cm³), and thesintering state thereof was good.

This secondary sintered body was heated from room temperature to 940° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 200 MPa, andthe temperature was held there for 1.5 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out. The relativedensity of the ‘tertiary sintered body’ was 100% (bulk density of 3.15g/cm³ and true density of 3.15 g/cm³), and the sintering state thereofwas good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 17

A MgF₂ powder being a main raw material was pulverized with the samepulverization conditions as in the above Example 11, and the pulverizedMgF₂ simple powder (main raw material: median diameter of about 4 μm andpurity of 99.9% by weight or more) was mixed with a sintering aid so asto obtain a starting raw material like the above Example 1.

On this starting raw material, uniaxial press molding was conducted withthe same molding conditions as in the above Example 1, and CIP moldingwas further conducted thereon so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 1000° C. at a fixed rate for 4 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the ‘secondary sintered body’ was 96.5%(bulk density of 3.04 g/cm³ and true density of 3.15 g/cm³), and thesintering state thereof was good.

This secondary sintered body was heated from room temperature to 940° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 200 MPa, andthe temperature was held there for 1.5 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out. The relativedensity of the ‘tertiary sintered body’ was 99.5% (bulk density of 3.135g/cm³ and true density of 3.15 g/cm³), and the sintering state thereofwas good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 18

A MgF₂ powder being a main raw material was pulverized with the samepulverization conditions as in the above Example 2, and the pulverizedMgF₂ simple powder (main raw material: median diameter of about 5 μm andpurity of 99.9% by weight or more) was mixed with a sintering aid so asto obtain a starting raw material like the above Example 1.

On this starting raw material, uniaxial press molding was conducted withthe same molding conditions as in the above Example 1, and CIP moldingwas further conducted thereon so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a hotpress furnace, and the temperature was held there for 8 hours. It wasthen raised to 1020° C. at a fixed rate for 4 hours, and the temperaturewas held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 10 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the ‘secondary sintered body’ was 95.2%(bulk density of 3.00 g/cm³ and true density of 3.15 g/cm³), and thesintering state thereof was good.

This secondary sintered body was heated from room temperature to 920° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 200 MPa, andthe temperature was held there for 2 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out. The relativedensity of the ‘tertiary sintered body’ was 99.0% (bulk density of 3.12g/cm³ and true density of 3.15 g/cm³), and the sintering state thereofwas good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 19

Using a MgF₂ simple powder pulverized with the same pulverizationconditions as in the above Example 11 (the pulverized raw material:median diameter of about 4 μm and purity of 99.9% by weight or more), asintering aid was added thereto so as to obtain a starting raw materiallike the above Example 1.

On this starting raw material, uniaxial press molding was conducted withthe same molding conditions as in the above Example 1, and CIP moldingwas further conducted thereon so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 800°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 960° C. at a fixed rate for 4 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the sintered body was 93.3% (bulk densityof 2.94 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

This secondary sintered body was heated from room temperature to 1100°C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 200 MPa, andthe temperature was held there for 0.75 hour.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out. The relativedensity of the sintered body was 97.6% (bulk density of 3.075 g/cm³ andtrue density of 3.15 g/cm³), and the sintering state thereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 20

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of1.5% by weight and pulverized for three weeks in the same manner as inthe above Example 1. The pulverized compound (median diameter of thepulverized compound of about 4 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using a hotpress furnace, and the temperature was held there for 5 hours. It wasthen raised to 1020° C. at a fixed rate for 4 hours, and the temperaturewas held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 10 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the sintered body was 97.1% (bulk densityof 3.06 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

This secondary sintered body was heated from room temperature to 940° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 200 MPa, andthe temperature was held there for 1.5 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out. The relativedensity of the sintered body was 99.8% (bulk density of 3.145 g/cm³ andtrue density of 3.15 g/cm³), and the sintering state thereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 21

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of1.5% by weight and pulverized for three weeks in the same manner as inthe above Example 1. The pulverized compound (median diameter of thepulverized compound of about 4 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 810°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 970° C. at a fixed rate for 4 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the sintered body was 93.7% (bulk densityof 2.95 g/cm³ and true density of 3.15 g/cm³), and the sintering statethereof was good.

This secondary sintered body was heated from room temperature to 1100°C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 200 MPa, andthe temperature was held there for 0.5 hour.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out. The relativedensity of the sintered body was 97.8% (bulk density of 3.08 g/cm³ andtrue density of 3.15 g/cm³), and the sintering state thereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 22

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of50% by weight and pulverized for two weeks in the same manner as in theabove Example 1. The pulverized compound (mean particle diameter of thepulverized compound of about 5 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 860°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 8hours. It was then raised to 1060° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout.

As HIP treatment, this secondary sintered body was further heated fromroom temperature to 1060° C. at a fixed rate for 6 hours in a nitrogengas atmosphere using a HIP furnace, while the gas pressure in thefurnace was set at 200 MPa, and the temperature was held there for 2hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out. The relativedensity of the tertiary sintered body was 97.2% (bulk density of 3.07g/cm³ and true density of 3.16 g/cm³), and the sintering state thereofwas good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 23

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of50% by weight and pulverized for two weeks in the same manner as in theabove Example 1. The pulverized compound (median diameter of thepulverized compound of about 5 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 850°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 8hours. It was then raised to 1020° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 95.2% (bulkdensity of 3.01 g/cm³ and true density of 3.15 g/cm³), and the sinteringstate thereof was good.

This secondary sintered body was heated from room temperature to 950° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a hotpress furnace, and the temperature was held there for 1.5 hours. In thisholding time, the press jig was driven to uniaxially apply a load of 50MPa for pressing.

Then, the pressing using the press jig and heating were stopped, and thetemperature was lowered by self-cooling (so-called furnace cooling) forabout 10 hours to a predetermined taking-out temperature of 100° C. orlower. After confirming that, the sintered body (tertiary sintered body)was taken out. The relative density of the tertiary sintered body was98.4% (bulk density of 3.11 g/cm³ and true density of 3.16 g/cm³), andthe sintering state thereof was good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

Example 24

A MgF₂ powder being a main raw material was mixed with a CaF₂ powder of90% by weight and pulverized for three weeks in the same manner as inthe above Example 1. The pulverized compound (median diameter of thepulverized compound of about 4 μm and purity of 99.9% by weight or more)was mixed with a sintering aid so as to obtain a starting raw materiallike the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 850°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 8hours. It was then raised to 1050° C. at a fixed rate for 4 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 97.2% (bulkdensity of 3.08 g/cm³ and true density of 3.15 g/cm³), and the sinteringstate thereof was good.

This secondary sintered body was heated from room temperature to 950° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 200 MPa, andthe temperature was held there for 1.5 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out. The relativedensity of the tertiary sintered body was 99.8% (bulk density of 3.165g/cm³ and true density of 3.17 g/cm³), and the sintering state thereofwas good.

Any of the evaluation results of the neutron moderation performance andmechanical strengths were good as shown in Table 2 of FIG. 10.

COMPARATIVE EXAMPLE Comparative Example 1

A high-purity MgF₂ raw material powder (median diameter: about 140 μmand purity of 99.9% by weight or more) was pulverized with the sameconditions as the three-day pulverization case of the above particlesize control test so as to obtain a MgF₂ powder (median diameter: about10 μm and purity of 99.9% by weight or more), to which a sintering aidwas added so as to obtain a starting raw material like the above Example1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 890°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 1170° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 91.7% (bulk density of2.89 g/cm³ and true density of 3.15 g/cm³), which was low. Whenobserving the inside of the sintered body under a microscope, there werea large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas bubbles in the sintering process since the median diameter of thecompound was about 10 μm, which was large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 2

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 1.5% by weight,and the compound was pulverized for five days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 8 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 1180° C. at a fixed rate for 2 hours, andthe temperature was held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 93.0% (bulk density of2.93 g/cm³ and true density of 3.15 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, somebubbles of several tens of μm in diameter were noticed. It wasconsidered that many bubbles remained since the median diameter of thecompound was about 8 μm, which was relatively large, and the secondarysintering temperature was 1180° C., which was too high.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 3

A high-purity MgF₂ raw material powder was mixed with a high-purity CaF₂raw material powder of 1.5% by weight, and the compound was pulverizedfor three days using the above alumina pot mill and alumina balls in thesame manner as in the Comparative Example 2. The pulverized compound hada median diameter of about 10 fu m and a purity of 99.9% by weight. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was pressed and molded at a pressure of 20MPa using a uniaxial press device in the same manner as in the aboveExample 1. This press molded body was put in the molding part of a coldisostatic pressing (CIP) device and CIP molding was conducted thereon ata molding pressure of 20 MPa at room temperature so as to form a CIPmolded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 900°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 5hours. It was then raised to 1050° C. at a fixed rate for 2 hours, andthe temperature was held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 92.1% (bulk density of2.90 g/cm³ and true density of 3.15 g/cm³), which was low. Whenobserving the inside of the sintered body under a microscope, there werea large number of fine bubbles of several μm in diameter and bubbles ofseveral tens of μm in diameter. It was considered that the former ofthese bubbles were foaming bubbles, and that the latter were part ofvoids which remained as bubbles since the median diameter of thecompound was about 10 μm, which was large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 4

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 1.5% by weight,and the compound was pulverized for three days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 10 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using auniaxial hot press furnace, and the temperature was held there for 5hours. It was then raised to 880° C. at a fixed rate for 2 hours, andthe temperature was held there for 1.5 hours. In each of these holdingtimes, the press jig was driven to uniaxially apply a load of 25 MPa forpressing.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 10 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 92.1% (bulk density of2.90 g/cm³ and true density of 3.15 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, someirregular-shaped bubbles were noticed. It was considered that manybubbles remained due to the insufficient degree of progress of sinteringsince the median diameter of the compound was about 10 μm, which waslarge, and the secondary sintering temperature was 880° C., which waslow.

Although the hot press method was used as the firing method in theprimary and secondary sintering steps, the insufficient sinteringproperty due to the coarse particle size of the compound and the lowsecondary sintering temperature could not be covered.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 5

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 20% by weight,and the compound was pulverized for three days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 10 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 840°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 8hours. It was then raised to 1200° C. at a fixed rate for 2 hours, andthe temperature was held there for 0.75 hour.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 92.1% (bulk density of2.90 g/cm³ and true density of 3.15 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, therewere a large number of bubbles of several tens of μm in diameter. It wasconsidered that these bubbles were part of voids which remained as largebubbles in the sintering process since the median diameter of thecompound was about 10 μm, which was large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 6

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 50% by weight,and the compound was pulverized for five days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 8 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 900°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 8hours. It was then raised to 1080° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 92.1% (bulk density of2.91 g/cm³ and true density of 3.16 g/cm³), which was low. Whenobserving the inside of the sintered body under a microscope, there werea large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas bubbles in the sintering process since the median diameter of thecompound was about 8 μm, which was relatively large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 7

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 50% by weight,and the compound was pulverized for three days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 10 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 860°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 8hours. It was then raised to 880° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 91.1% (bulk density of2.88 g/cm³ and true density of 3.16 g/cm³), which was low. Whenobserving the inside of the sintered body under a microscope, there werea large number of irregular-shaped bubbles. It was considered that manybubbles remained due to the insufficient degree of progress of sinteringsince the median diameter of the compound was about 10 μm, which waslarge, and the secondary sintering temperature was 880° C., which waslow.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 8

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 90% by weight,and the compound was pulverized for five days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 8 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 910°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 9hours. It was then raised to 1080° C. at a fixed rate for 2 hours, andthe temperature was held there for 2 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 92.1% (bulk density of2.92 g/cm³ and true density of 3.17 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, therewere a large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas many bubbles in the sintering process since the median diameter ofthe compound was about 8 μm, which was relatively large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 9

A high-purity MgF₂ powder (median diameter: about 8 μm and purity of99.9% by weight or more), which was obtained by pulverizing with thesame conditions as the five-day pulverization case in the above particlesize control test, was mixed with a sintering aid so as to obtain astarting raw material in the same manner as in the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 730°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 6hours. It was then raised to 880° C. at a fixed rate for 2 hours, andthe temperature was held there for 3 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 90.5% (bulkdensity of 2.85 g/cm³ and true density of 3.15 g/cm³).

This secondary sintered body was heated from room temperature to 1160°C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 91.7% (bulk density of2.89 g/cm³ and true density of 3.15 g/cm³), which was low. Whenobserving the inside of the sintered body under a microscope, there werea large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas many bubbles in the sintering process since the median diameter ofthe compound was about 8 μm, which was relatively large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 10

A high-purity MgF₂ powder (median diameter: about 10 μm and purity of99.9% by weight or more), which was obtained by pulverizing with thesame conditions as the three-day pulverization case in the aboveparticle size control test, was mixed with a sintering aid so as toobtain a starting raw material in the same manner as in the aboveExample 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 890°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 3hours. It was then raised to 1180° C. at a fixed rate for 2 hours, andthe temperature was held there for 1 hour.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 91.4% (bulkdensity of 2.88 g/cm³ and true density of 3.15 g/cm³), which was low.

This secondary sintered body was heated from room temperature to 740° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 93.0% (bulk density of2.93 g/cm³ and true density of 3.15 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, therewere a large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas many bubbles in the sintering process since the median diameter ofthe compound was about 10 μm, which was large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 11

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 1.5% by weight,and the compound was pulverized for five days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 8 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 730°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 6hours. It was then raised to 880° C. at a fixed rate for 2 hours, andthe temperature was held there for 3 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 91.1% (bulkdensity of 2.87 g/cm³ and true density of 3.15 g/cm³).

This secondary sintered body was heated from room temperature to 1170°C. at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 92.4% (bulk density of2.91 g/cm³ and true density of 3.15 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, therewere a large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas many bubbles in the sintering process since the median diameter ofthe compound was about 8 μm, which was relatively large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 12

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 1.5% by weight,and the compound was pulverized for three days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 10 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 890°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 3hours. It was then raised to 1190° C. at a fixed rate for 2 hours, andthe temperature was held there for 0.5 hour.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 92.1% (bulkdensity of 2.90 g/cm³ and true density of 3.15 g/cm³).

This secondary sintered body was heated from room temperature to 740° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 94.0% (bulk density of2.96 g/cm³ and true density of 3.15 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, therewere a large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas many bubbles in the sintering process since the median diameter ofthe compound was about 10 μm, which was large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 13

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 20% by weight,and the compound was pulverized for five days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 8 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 730°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 6hours. It was then raised to 880° C. at a fixed rate for 2 hours, andthe temperature was held there for 3 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 91.1% (bulkdensity of 2.87 g/cm³ and true density of 3.15 g/cm³).

This secondary sintered body was heated from room temperature to 730° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 92.7% (bulk density of2.92 g/cm³ and true density of 3.15 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, therewere a large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas many bubbles in the sintering process since the median diameter ofthe compound was about 8 μm, which was relatively large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 14

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 20% by weight,and the compound was pulverized for three days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 10 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 890°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 3hours. It was then raised to 1190° C. at a fixed rate for 2 hours, andthe temperature was held there for 1 hour.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 92.4% (bulkdensity of 2.91 g/cm³ and true density of 3.15 g/cm³).

This secondary sintered body was heated from room temperature to 730° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 94.6% (bulk density of2.98 g/cm³ and true density of 3.15 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, therewere a large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas many bubbles in the sintering process since the median diameter ofthe compound was about 10 μm, which was large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 15

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 50% by weight,and the compound was pulverized for five days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 8 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 730°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 6hours. It was then raised to 900° C. at a fixed rate for 2 hours, andthe temperature was held there for 3 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 92.1% (bulkdensity of 2.91 g/cm³ and true density of 3.16 g/cm³).

This secondary sintered body was heated from room temperature to 760° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 94.3% (bulk density of2.98 g/cm³ and true density of 3.16 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, therewere a large number of bubbles of several μm and several tens of μm indiameter. It was considered that the former of these bubbles werefoaming bubbles, and that the latter were part of voids which remainedas many bubbles in the sintering process since the median diameter ofthe compound was about 8 μm, which was relatively large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 16

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 50% by weight,and the compound was pulverized for three days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 10 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 910°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 3hours. It was then raised to 1200° C. at a fixed rate for 2 hours, andthe temperature was held there for 0.5 hour.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 92.7% (bulkdensity of 2.93 g/cm³ and true density of 3.16 g/cm³).

This secondary sintered body was heated from room temperature to 760° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 95.3% (bulk density of3.01 g/cm³ and true density of 3.16 g/cm³). When observing the inside ofthe sintered body under a microscope, there were a large number ofbubbles of several μm in diameter. The sizes of these bubbles were thoseof foaming bubbles. It was considered that foaming briskly occurred inthe sintering process since the median diameter of the compound wasabout 10 μm, which was large, and the heating temperatures in theprimary and secondary sintering were relatively high.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 17

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 90% by weight,and the compound was pulverized for five days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 8 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 730°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 7hours. It was then raised to 900° C. at a fixed rate for 2 hours, andthe temperature was held there for 3 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 92.4% (bulkdensity of 2.93 g/cm³ and true density of 3.17 g/cm³).

This secondary sintered body was heated from room temperature to 760° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 93.7% (bulk density of2.97 g/cm³ and true density of 3.17 g/cm³), which was relatively low.When observing the inside of the sintered body under a microscope, therewere a relatively large number of bubbles of several μm and several tensof μm in diameter. It was considered that the former of these bubbleswere foaming bubbles, and that the latter were part of voids whichremained as many bubbles in the sintering process since the mediandiameter of the compound was about 8 μm, which was relatively large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

Comparative Example 18

A high-purity MgF₂ raw material powder (main raw material: mediandiameter of about 140 μm and purity of 99.9% by weight or more) wasmixed with a high-purity CaF₂ raw material powder (median diameter:about 140 μm and purity of 99.9% by weight or more) of 90% by weight,and the compound was pulverized for three days using the above aluminapot mill and alumina balls. The pulverized compound had a mediandiameter of about 10 μm and a purity of 99.9% by weight or more. Asintering aid was added to the pulverized compound so as to obtain astarting raw material like the above Example 1.

This starting raw material was filled into a mold form of a pressdevice, and compressed and molded at a uniaxial press pressure of 20 MPain the same manner as in the above Example 1. This press molded body wasput in the molding part of a cold isostatic pressing (CIP) device andCIP molding was conducted thereon at a molding pressure of 20 MPa atroom temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 910°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 4hours. It was then raised to 1200° C. at a fixed rate for 2 hours, andthe temperature was held there for 0.5 hour.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body (secondary sintered body) was takenout. The relative density of the secondary sintered body was 93.1% (bulkdensity of 2.95 g/cm³ and true density of 3.17 g/cm³).

This secondary sintered body was heated from room temperature to 760° C.at a fixed rate for 4 hours in a nitrogen gas atmosphere using a HIPfurnace, while the gas pressure in the furnace was set at 100 MPa, andthe temperature was held there for 4 hours.

Then, while gradually decreasing the gas pressure in the furnace, theheating was stopped, and the temperature was lowered by self-cooling(so-called furnace cooling) for about 10 hours to a predeterminedtaking-out temperature of 100° C. or lower. After confirming that, thesintered body (tertiary sintered body) was taken out.

The relative density of the sintered body was 94.6% (bulk density of3.00 g/cm³ and true density of 3.17 g/cm³). When observing the inside ofthe sintered body under a microscope, there were a large number ofbubbles of several μm in diameter and some bubbles of several tens of μmin diameter. The sizes of the former were those of foaming bubbles. Itwas considered that foaming briskly occurred in the sintering processsince the heating temperatures in the primary and secondary sinteringwere relatively high. The latter varying in size and shape were part ofvoids which remained as bubbles in the sintering process since themedian diameter of the compound was about 10 μm, which was large.

In the evaluation results of the neutron moderation performance andmechanical strengths, as shown in Table 3 of FIG. 11, some insufficientlevels of neutron moderation performance and mechanical strengths werenoticed.

[Comparative Material 1]

The above CaF₂ powder being a secondary raw material (median diameter ofabout 6 μm), which was prepared in the same manner as in the aboveExample 12 as a starting raw material, was filled into a mold form of apress device, and compressed and molded at a uniaxial press pressure of20 MPa in the same manner as in the above Example 1. This press moldedbody was put in the molding part of a cold isostatic pressing (CIP)device and CIP molding was conducted thereon at a molding pressure of 20MPa at room temperature so as to form a CIP molded body.

Preliminary sintering was conducted on this CIP molded body at 650° C.for 6 hours in an air atmosphere so as to obtain a preliminary sinteredbody.

This preliminary sintered body was heated from room temperature to 880°C. at a fixed rate for 6 hours in a nitrogen gas atmosphere using anatmospheric firing furnace, and the temperature was held there for 6hours. It was then raised to 1100° C. at a fixed rate for 2 hours, andthe temperature was held there for 1.5 hours.

The heating was then stopped, and the temperature was lowered byself-cooling (so-called furnace cooling) for about 20 hours to apredetermined taking-out temperature of 100° C. or lower. Afterconfirming that, the sintered body was taken out.

The relative density of the sintered body was 95.3% (bulk density of3.03 g/cm³ and true density of 3.18 g/cm³).

The mechanical strengths were good, but a large quantity of fastneutrons remained after moderation, that is, the neutron moderationperformance was insufficient.

INDUSTRIAL APPLICABILITY

It is possible to be used as a moderator to restrict the radiationvelocity of radioactive rays of every kind such as neutrons.

The invention claimed is:
 1. A MgF₂ system fluoride sintered body for aradiation moderator, consisting of MgF₂ having a compact polycrystallinestructure with a bulk density of more than 3.07 g/cm³ and a relativedensity of more than 97.5%, which has a bending strength of 12 MPa ormore and 59 MPa or less and a Vickers hardness of 100 or more and 242 orless as regards mechanical strengths.
 2. A method for producing a MgF₂system fluoride sintered body for a radiation moderator, comprising thesteps of: pulverizing a high-purity MgF₂ raw material to control theparticle size, so as to allow the maximum particle diameter in aparticle size distribution to be 50 μm or less, the shape of theparticle size distribution curve to be of sub-1-peak type or 1-peaktype, and the median diameter to be 6 μm or less; adding 0.02-1% byweight of a sintering aid to the particle-size-controlled raw materialto mix; molding the compound at a molding pressure of 5 MPa or moreusing a press molding device; molding the press molded article at amolding pressure of 5 MPa or more using a cold isostatic pressing (CIP)device; conducting preliminary sintering by heating the CIP moldedarticle in a temperature range of 600° C.-700° C. in an air atmosphere(preliminary sintering step); conducting atmospheric sintering orpressure sintering by heating in a temperature range from (Tn−100)° C.to (Tn)° C. when the starting temperature of foaming of the preliminarysintered body is (Tn)° C., in an air atmosphere or in an inert gasatmosphere or in a vacuum atmosphere (primary sintering step); andforming a sintered body having a compact structure by heating in atemperature range of 900° C.-1150° C. under atmospheric pressure orunder pressure in the same atmosphere as the preceding step (secondarysintering step), and wherein the sintered body consisting of MgF₂ havinga compact polycrystalline structure with a bulk density of more than3.07 g/cm³ and a relative density of more than 97.5%, which has abending strength of 12 MPa or more and a Vickers hardness of 100 or moreas regards mechanical strengths.
 3. The method for producing the MgF₂system fluoride sintered body for a radiation moderator according toclaim 2, further comprising the steps of: cooling the sintered bodyafter the secondary sintering step; and conducting tertiary sintering byreheating in a temperature range of 900° C.-1150° C. under pressure inan inert gas atmosphere or in a vacuum atmosphere.
 4. The method forproducing the MgF₂ system fluoride sintered body for a radiationmoderator according to claim 2, wherein the inert gas atmosphere in thetwo sintering steps (primary and secondary sintering steps) comprisesone kind of gas or a mixture of plural kinds of gases, selected fromamong nitrogen, helium and argon, and hot molding work is conducted inthe heating process using a hot press furnace or a hot isostaticpressing furnace.
 5. The method for producing the MgF₂ system fluoridesintered body for a radiation moderator according to claim 3, whereinthe inert gas atmosphere in the three sintering steps (primary,secondary and tertiary sintering steps) comprises one kind of gas or amixture of plural kinds of gases, selected from among nitrogen, heliumand argon, and hot molding work is conducted in the heating processusing a hot press furnace or a hot isostatic pressing furnace.
 6. Themethod for producing the MgF₂ system fluoride sintered body for aradiation moderator according to claim 2, wherein in the two sinteringsteps (primary and secondary sintering steps), in a vacuum atmosphere ofless than 100 Pa, using a hot press furnace, hot molding work isconducted in the heating process.
 7. The method for producing the MgF₂system fluoride sintered body for a radiation moderator according toclaim 3, wherein in the three sintering steps (primary, secondary andtertiary sintering steps), in a vacuum atmosphere of less than 100 Pa,using a hot press furnace, hot molding work is conducted in the heatingprocess.